A guard is stationed at the entrance of a bridge. He is tasked to shoot anyone who tries to cross to the other side of the bridge, and to turn away anyone who comes in from the opposite side of the bridge. You are on his side of the bridge and want to escape to the other side.
Because the bridge is old and rickety, anyone who tries to cross it does so at a constant speed, and it always takes exactly 10 minutes to cross.
The guard comes out of his post every 6 minutes and looks down the bridge for any people trying to leave, and at all other times he sits in his post and snoozes. You know you can sneak past him when he’s sleeping, but the problem is that you won’t be able to make it all the way to the other side of the bridge before he sees you (since he comes out every 6 minutes, but it takes 10 minutes to cross).
One day a brilliant idea comes to you, and soon you’ve successfully crossed to the other side of the bridge without being shot. How did you do it?
Immediately after the guard goes back to his post upon checking the bridge, you start to cross the bridge. Just before the 6 minutes is up, turn around and start walking back towards the guard.
The guard will come out and notice you and assume that you are a visitor coming from the other side of the bridge. He will go back to his post since he will not turn you away until you reach him.
Once the guard has gone back, immediately turn back around and walk to the other side of the bridge. Since you had walked almost 6 minutes previously, you will reach the other end of the bridge in about 4 minutes time – before the guard comes out of his post again.