This.
Quite often, I think of pretty nice questions whilst in the shower, or just assignment problems if I'm given one.
Once I spent HOURS on a differential equation problem modelling heat seeking missiles and was on the brink of giving up (tears were shed), so I went to have a shower to refresh my mind. At that point, I was pretty darn frustrated.
As the water was getting warmer, the solution just suddenly hit me (because the solution to the problem required recognition of changing temperatures as the missile travels through the air) and I clambered out (nobody was home thankfully), jotted down the basic steps for the solutions, then got back in and enjoyed the rest of my shower.