How to get back at my brother?

Okay so me and my brother are super close. I'm adopted, he's biological. I'm 13 and he's 15. Weve been through alot together and he's gotten me through everything. (,self harm, and much more). Anyways we typically get into "battles" lol cause were just mature like that. Well I found this slime crap that we had when we were like 5 and 7. I started throwing it around and told him to catch. I laugh at everything btw which makes him laugh:) and it was super funny we were playin catch and he was really grossed out by it and paranoid that it would get somewhere. I was just enjoying myself:) then game over, he put it IN MY HAIR! I have think long hair so I had to rinse the crap out right away...now it's game on hahah what an I do to get back at him?? I usually go to bed earlier then him so I cant really do anything while he's sleeping..hah. Any ideas? Thanks! Just havin alittle fun:)