A farmer has a sack of grain, a chicken and a fox. He has to get them across the river and he can't go around or fly but he does have a boat. He can only take one thing at a time across the river. How can he get them across?
Take the chicken over and leave the fox and the grain. Go get the fox drop it off and pickup the chicken. Take the chicken back across and pickup the grain and leave the chicken. Drop the grain off with the fox and take the chicken over now all of them are on one side!
You can feel it, but you can't touch it. You can hear it, but you can't see it. What is it?