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?