This is the oldest riddle that I have heard.
Once there was a man who is travelling with a Tiger , goat and grass.
He needs to cross the river that can carry the maximum of two i.e Man + one of Tiger goat or grass. But the problem is "If he takes lion with him, the goat will eat grass and if he takes grass with him, the lion will eat goat".
How can he ensure safe delivery of all three?
It can be done in four steps.
1. Men & goat to the other end. Then the man comes back.
2. Man & lion to the other end. Then the man comes back with the goat. 3. Man & grass to the other end. The man comes back alone
4. Man & goat to the other end.