I came up with this yesterday.
Sink or float? - physics (buoyancy)
Sink or source? - EE (current)
Think or swim? - Finance (investment?)
Anyone want to add more?
Sink or toilet - AA (Alcoholics Anonymous)
Wait... I don't think I understand the game.
biocow:
Sink or toilet - AA (Alcoholics Anonymous)Wait... I don't think I understand the game.
Haha! 
Think (or Thwim) - IBM
biocow:
Sink or toilet - AA (Alcoholics Anonymous)Wait... I don't think I understand the game.
Ha ha ha! Good one! XD
AWOL:
Think (or Thwim) - IBM
I didn't know IBM has that too? 
Sync or async.
Graynomad:
Sync or async.
Sync or async - informatics (communication protocols) 
tan or atan (triangles :D)
tan or skin tancer (beach)
Haha this thread is getting silly
2B || !2B
(deleted)
spycatcher2k:
2B || !2B = 0xFF
XD XD XD
Boolean algebra
Zinc or Tin ? (metallurgy!)
Sink or source, just like an Arduino output pin.
Think or stay stupid ? 