Any ideas for a cool prank?

Well me and my ocusins practically have like a prank war.... so i need really good ideas for pranks...you can make them up or tell me some you used in the past....an example is...like one time they barked everytime i said the word "the" and i pretended i had some medicine that made me fart...and i played a farting noise every five minutes...and i didnt tell them until they left...so can you please give me good ideas! 10points to the best answer!