How to destroy someone's car without leaving any evidences of sabotage?

i dont mean to explode car!!!
to screw the engine or to damage mechanical parts that are expensive to repair or replace.
Guys this is a serious question. The best answer will be implemented. And i'm not stupid and dumb enough to post an evidence of my crime over the internet. This is something a friend of a friend of a friend of mine wants to do?
This is something a friend of a friend of a friend of mine wants to do!!!!!

I'm already considering a few tips. Thanks to them who help me.

Answer: 
Heres my ideas..... -Try pouring drain cleaner into their engine oil inlet. be sure it contains primarily sulfuric acid. It will partially blend with the oil and be the same color, but will eat destroy the seals, and create havoc. Be carefull if you spill it it, will disolve some plastic in the engin bay which may be evident. -Try simply pouring more oil into the engine, add like 2 gallons extra. Too much oil will raise the oil pressure substancially when the engine heats up which may cause gasketsand seals to fail. -Stuff a bunch of easily flamable material into the engine bay near the exhaust manifold. Maybe go further and spread it elswhere aroudn the engine bay so the fire spreads faster. -Take bolt cutters and cut the vehicles wiring harness at the point that it comes out of the firewall. total pain in the dick to repair. -Drop a solid metal object down the spark plug well and into the combustion chamber -loosten the oil drain nut -Putting sand in the timing belt/chain area could create havoc, but its difficult to get sand in there. -remove a spark plug wire, and overtighten a spark plug untill it strips out and spins freely...bam 2500+ in damage Thats all i can think of now