A farmer needs to transport a fox, a chicken, and a grain across a river. However, he can only take one item at a time in his boat. If left alone together, the fox will eat the chicken, and the chicken will eat the grain. To solve this, the farmer first takes the chicken across the river, leaving the fox and grain together. He then returns alone, takes the fox across, and brings the chicken back. Finally, he leaves the chicken on the original side, takes the grain across, and returns for the chicken.