This is the best trick for classes where computers are used a lot by morons. First what you do is go to the Apple Menu on a mac, go down to "control panels" and open up the alert sound controls (usually called "sound"). You have to do this atlunch or after school when no one's around. Anyway, click "new sound" and record something evil and fun, like a monkey noise, or loud swearing. Make sure to disguise your voice somehow. Then save it. Scroll down and select it. It will play automatically and by selecting it you've made it the Mac alert sound! So now when classmates are on the computer, it will play if they turn up the volume or an alert window pops up. This is a good one to do for those cranky 80-something teachers who freak out about everything. Have fun with this one!!