Install the Blue Screen of Death screen-saver on someone's computer.

 797

3  

Hide all of the desktop icons on someone's computer and replace the monitor's wallpaper with a screen-shot of their desktop.

 861

0  

Find a sleeping person, fill their hand with shaving cream and then tickle their nose.

 772

0  

Put tape over the optical sensor of someone's mouse.

 831

1  

Paint a bar of soap completely with clear nail polish so it won't suds up.

 731

1