I do this one a lot on my cousin and its really funny watching her reaction.
What you need
•someone to annoy
•a voice
•a brain would help

What to do
Go up to the person you are trying to annoy.(better if you run or skip happily)
Say:
"Guess what!?" In a really excited voice. The person will then get all excited.

They will probably say:
"What!?" You reply saying
"Guess." The person should try and guess. After about three guess ask them if they want to give up. Unless they're some person who loves to guess they will give up!!

Then you say something like:
"Oh I just really needed to pee." Or
"Nothing." After saying that they will push you away.

This is the part when you either run away or laugh and point. I love to laugh and point.