Okay, here is one more. Again, no tricks. Just use your logic.
You are alone in a room.
In the room there are two lengths of fuse (like the sort of thing used to light dynamite or a firecracker), a box of matches and a pair of scissors. Each fuse is about five feet long, but not exactly. You know that each fuse burns for exactly one hour. But you also know that they burn unevenly (so that it could burn up four of the five feet in the first ten minutes but then take the next fifty minutes to burn the last foot, or in some other way, you don't know). You also know that each fuse is different (so while they both burn unevenly, they both burn at different rates). You don’t know the rate at which either fuse will burn, just that they each last exactly one hour.
The only way you can get out of the room is by measuring 45 minutes, exactly. There are no watches, cell-phones, clocks, sundials, miniature stone henges or any other time telling devices available to you. Just what you have, the two fuses, a box of matches and a pair of scissors (which can cut the fuses into any lengths you like).
How do you measure 45 minutes?