For this prank you will need:
Access to a toilet (At home, a dorm, hotel, school, office, etc.)
Food dye
This prank works best if the toilet has a tank in the back. Most homes have tank toilets, but a lot of public places have toilets that just have a pipe connected to it.
If there is a tank:
Open up the top. Put in food coloring (any color). You don't need a whole lot, becuase food dye is concentrated, but don't be afraid to put in lots. Then close the lid of the tank. The next time someone flushes the toilet, the new water that comes down will be the color of the food dye.
-Blue makes it look like there are chemicals in the water (like an airplane)
-Yellow looks like the tank was just filled with pee insead of water.
-Green makes the water look like algea is living in it
-Red is scary (especially if the coloring isn't completely mixed withthe water): It looks like blood.
Anyways, this works very well.
If there is no tank (this works with tank toilets too):
Put the food dye in the water in the toilet. This will work only once (With the dye in the tank, it can go another flush before all the dye is out), but it is still funny to see people act weird when they see it. People will be very scared if they see red in the toilet...
Another toilet prank:
Put a small plant in the toilet. Simple as that. It will cause confusion (Note: I have never tried that one)
Yet another one:
This works best if you can get it to stay overnight:
Put a jell-o or other geletin mix in the toilet. You will probably have to use more than one small pack. If you can get clear geletin mix, then people might pee or poop on top without realizing it. It still will be funny if you use flavored jell-o, because just think: Jell-O in the toilet?
I am not sure about other operating systems, but this definately works on windows XP. (The directions are for XP, feel free to add directions for other OS's.)
Go to control panel. If you see a menu asking you to pick a category, select "Printers and other Hardware". If you see a big list of things, then go right to "Mouse." In the buttons tab, select "Switch primary and secondary buttons." This will cause the person to "right click" when they hit the left mouse button, and vice versa. If you go to the "pointers" tab, you can change the default mouse pointer to something that usually means something else (like the hourglass). If you go to the "pointer options" tab, you can change the pointer speed (making the cursor go far when you move the mouse only a little), and display pointer trails (make them long) to confuse the user when they use the mouse.
DON'T FORGET TO APPLY SETTINGS! Don't close the window untill you have applied the settings (or you can click okay).
Also, there are many pranks at:
http://www.pranksite.com/