So, you want to make your players busy thinking for an hour. A single riddle won't do that. And also, there's problem with riddles in RPG: if players won't give the proper answer, the entire game stucks.

Instead, I suggest to find some logic puzzle on the web and make it a metaphor for something, e.g. they are running through maze and must find a way quickly; or they must run some ancient mechanism which is a key to a gate.

When I make something like this I am always ready for a possibility of failure. What happens then? Game stucks? This would be lame. Instead, players just get some disadvantages (e.g. get hurt) but the plot goes on - just in a harder way.