This is my favorite toilet prank so far. When your victim flushes the toilet, the water goes down, but the water stays yellow. This prank will make it seem that the toilet is broken, or the pipes might be clogged. Another reason Why I like this prank is because you can do it in less than thirty seconds.
Step 1: Ingredients
Food coloring (I like yellow, but I guess you could do another color)
Step 2: Fill Your Syringe
This is pretty self explanitory. If you are going to use a syringe, fill it with a few drops of food coloring.
Step 3: Fill the Tank
Now get your few drops of food coloring in the TANK of your toilet. After a minute it will mix into an even yellow. If you want it to mix faster I guess you could use a spoon.
Let it set for a minute before you victim comes, and when he does, he might not notice it at first, but after a while he will.