There's a great opportunity to play pranks with your Mac this April Fool's Day.

If you're into computer pranks, you've probably already written AppleScripts with scary messages ("Are you sure you want to delete the volume Macintosh HD and all backups?"), or replaced someone's bookmarks with links to shock sites. Now there's a new kind of pranking going on, across the Internet, and it doesn't involve any form of computer trespass.

In the past few months, websites that allow strangers to video conference with each other have been getting a lot of media attention. You may have heard of sites like ChatRoulette: users simply click a button to be connected, by video, to a random stranger. Once the conversation gets old (or if someone sees or hears something they don't like), users click again to find someone else. In a typical session, users can connect to hundreds of people in an hour.

We gave this fellow on the right a good laugh. (We've made other people scream.)

It's as if your targets will be lining up for you.

Whether you choose to make people laugh or make people scream is up to you. Just be forewarned: Some users of these sites have their own ideas about what videoconferencing is for. While you're out pranking, you may find someone wanking.

With some thought, you can trick hundreds of people per hour on your Mac. (And if you get hooked, you can continue past April Fool's Day.)

WHAT YOU NEED

To use these sites:
Adobe Flash (may depend on the site, but we used Flash 10)
A web browser (we used Safari 4)

To videoconference:
A web camera
A microphone

To add effects or transmit QuickTime video:
Allocinit CamTwist (free, allocinit.com)

To transmit QuickTime audio:
Cycling '74 Soundflower (free, cycling74.com)

To convert FLV video to mp4:
Handbrake (free, handbrake.fr)

1. Connect

Though ChatRoulette is the videoconferencing site that has been getting all the media attention lately, we've been having a lot more fun, and have met more celebrities, on ShufflePeople. There are probably a handful of other sites that do the same thing. They're probably also built on Flash, just like ChatRoulette and ShufflePeople.

Once you connect to one of these sites, you need to give the site permission to use your microphone and camera before people can see you. You may also need to make sure the Flash Player knows which camera and mic to look for, and set the volume of your mic.

In case you can't figure out how to change these settings, there's a way to do it in every Flash program: right-click on the Flash content, and choose "Settings..." On the Privacy tab, Make sure "Allow" is checked, and click the "Remember" box so you don't have to authorize it every time you connect. On the Microphone tab, make sure the correct mic is selected, and adjust the Record Volume slider as necessary. Click "Reduce Echo" if you prefer. In the Camera tab, make sure the correct camera is selected.

Click Close when you're done.

The Privacy tab has an eye on the monitor. The Microphone and Camera tabs look like, well, a mic and a webcam.

Click the start button to connect to a stranger. Now that you're connected have you given any thought to what pranks you might play?

If you like making people laugh, try dressing up in a chicken costume, or some other ridiculous garb. Come up with a zany character, and work on your improv comedy skills. If you're good at it, people will watch you longer.

To shock people, mix some corn syrup and grenadine and drizzle it all over your face to make it look like you're bleeding. You can even put it in your mouth, and add some white chewing gum squares, then make it look like you're spitting teeth into a bucket. Or barf up some pea soup.

If you're feeling especially mean-spirited, you can try insulting people before they can reach the "next" button.

2. Show them video

You may eventually want to show people something other than video from your web cam. With AllocInit CamTwist, a free download, you can show QuickTime videos to the other person.

CamTwist pretends it is a webcam, so once you've got it installed and running, you'll have to tell the chat website to look for CamTwist instead of your regular camera. (See step 1, above.) Optionally, you can install Cycling '74 Soundflower if you would like the sound in the QuickTime videos to go directly to your viewers, instead of through your speakers and into your mic. If you do that, tell CamTwist to send QuickTime audio to Soundflower by going to CamTwist > Preferences... and clicking the General tab. For "Audio Input Device" select "Soundflower (2ch)." You won't be able to hear your QuickTime videos, but your audience will hear them better. (You'll also have to tell the Flash program to get its audio from Soundflower. See step 1, above.)

Here's the part where we bait them. Use CamTwist to show your victims something they'll want to watch. This video here is of poor quality, but it still tends to fool people. We had to use a zoom effect to make sure it filled the frame. Read on for the switch.

To show a video, select "Movie" as the video source and click the button that says "no movie selected" to load a movie into the playlist. (You can drag a video file directly to the playlist.) Be sure to look at it in Tools > Preview to see if it fills the entire frame. If it doesn't, some people may notice right away they're looking at a video. Make it larger by double-clicking on Zoom in the list of effects, and adjusting the slider as necessary.

3. Show them other things

In addition to still images, CamTwist lets you show your desktop, or your webcam. You can also add effects to any of those, and save presets so you can quickly switch from one source to another.

You can display still images, but most people recognize these right away, and will move on to the next person. Drag the images, or the folder that contains them, to the list in order to make them appear in the slideshow. The number next to "Delay:" is the number of seconds it waits before switching to the next image. Hit the Pause button if you want to switch between images on your own by clicking.

Won't someone think of the kittens? We can switch to other images, some funny, some gross, by clicking on their file names.

You can use images to display a funny message, to shock people, or to make crude animation. If your images are vertical, you may want to try experimenting with the Zoom effect to fill the frame more.

Alternatively, if you're just looking for interesting or weird effects to use, select Webcam as the video source, and experiment with the various effects. (Msa_badtv is one of our favorites.)

4. Switch!

Our favorite way to use CamTwist is to use QuickTime videos to bait and switch. We'll start by showing something appealing, such as girls dancing. The victim will often hoot, holler, whoop, and offer words of encouragement. If he's in college, he'll call over his dorm room buddies. Then we'll switch to a video of something off-putting, like a dude playing with his belly fat, and watch and listen to the screams and howls.

If you think you'll be doing a lot of switching, you may find it faster to switch between finished setups, especially if you switch between sources that have effects and those that don't. To create a setup, select a source, add effects and adjust their settings (if necessary), and click the "Save Setup" button. Be sure to uncheck "Don't include video source" if you want the setup to include the video you're playing.

CamTwist lets us quickly and easily switch between showing strangers video clips of pretty dancing girls and chubby hairy dudes, which is obviously what AllocInit was thinking about when they designed it.

Keep the CamTwist window off to one side as you connect to the chat site. When you're ready to switch, quickly double-click on the setup you'd like people to see. It's that simple!

5) Download some video

There are plenty of videos already available online that are ideal for this sort of pranking. You can find all kinds of bizarre things on video sites like YouTube, and because they're made by normal people, they are great for tricking people.

What we find most effective is a video shot indoors with a stationary camera, with no talking (unless the person in the video talks really fast, for a comical effect). That sort of video looks like it was made on a webcam, and will fool a lot more people into thinking they're looking at something other than a video. Find such a video, and download it.

Oh, look! Here's a YouTube video of someone dancing in front of a stationary camera, with poor lighting and bad color balance. It's perfect! Thanks to Safari's Activity window, we can option-double click on the source file, and we'll have a copy of our own for baiting.

While there are no on-screen controls in YouTube for downloading videos, Safari provides a way: its Activity window. Navigate to Window > Activity to see every file being sent to your browser. (If you have multiple windows or tabs open, use the triangle controls on the left of each page name to hide the files for every page except the video site page. Look in the list of files for the largest file you can find (close to a megabyte, or larger). That one is most likely the video. Option-double click on it to download it.

In case you download an .FLV file, you will have to convert it to a format QuickTime understands before CamTwist can use it. We had pretty decent luck using Handbrake, a free download from handbrake.org. Launch Handbrake, and locate the file you downloaded. Choose a destination for the resulting file, and click the start button.

Tell CamTwist about the converted file (See step 2, above), and you're on your way to using the strange stuff you found online to get funny reactions from other people.

By the way, if you're looking for inspiration, lots of people have been uploading their chat prank exploits to YouTube. Happy April Fool's Day.