The funniest thing I have done is when I pranked my parents by replacing toothpaste with mayonnaise in the toothpaste tube and then set it up in the bathroom. You can also view other funny comments on forums like www.sodahead.com and boards.straightdope.com.
My math teacher said that kids laugh at everything, so he held up a calculator, and said, "Calculator." People started laughing. Then he picked up his pencil and said, "Pencil." He put the pencil upside down and said, "Upside down pencil." Everyone was laughing so hard. It wasn't supposed to be funny, but it was hilarious.
In Tae Kwan Do class I saw a toddler interrupt our class and walk in their unassigned and said hi in her squeaky little voice. And then Mr. Miller, our instructor carried her out of the class because she was too little and wasn't attended. But she was like 2 years old and didn't know better but that was hilarious.