You're a farmer with a boat, on one side of a river. With you is your fox, your chicken, and a sack of grain. You can only take one with you on your boat to cross. If you leave the chicken alone with the grain, you can kiss your grain goodbye. If the chicken and the fox are left alone together, let's just say you'll have a happy fox. How do you get all three across?

To start with, you have the chicken, the fox, and the grain on one side. (u=you, the rest should be obvious)
Left: {F,G,C,u}
Right: {}

First, you take the chicken across the river. The fox doesn't want the grain, so it should be safe.
Left: {F,G}
Right: {C,u}

Then, you go back across the river yourself.
Left: {F,u,G}
Right: {C}

Take either the fox or the grain, it doesn't matter which. For the purpose of this example, I'm taking the fox.
Left: {G}
Right: {F,u,C}

Now take the chicken back with you to the original side.
Left: {G,u,C}
Right: {F}

Take the grain over to hang out with the fox.
Left: {C}
Right: {F,G,u}

Go back to the side with the chicken.
Left: {C,u}
Right: {F,G}

Take the chicken to the right side, and you're done.
Left: {}
Right: {F,G,C,u}
This was a game in an old Macintosh game pack that I sorely miss.