Online, however, they may accept advertising, if it is unobtrusive, relevant and fun.

ECONOMIST: Internet advertising

"It's our way of having a fun PSA built into the game that is relevant and fits in with the subject matter, " Mustard said.

CNN: It's Obama vs. Romney in new mobile game

Mersenne primes are most relevant to number theory, but most participants join Gimps for the fun of having a role in real research - and the chance of finding a new Mersenne prime.

BBC: NEWS | Science/Nature | Largest Prime Number discovered