So if he brings the fox first.
Then the swan will eat the grain.
If he brings the grain first the fox will eat the swan.
So he takes over the swan and goes back for grain.
When he takes over for the the grain.
Then he takes grain over he takes the swan back to the other side again
Then he brings the fox back over again knowing that he cannot eat the grain
Leaving the fox and the grain together he brings over the swan again
When he reaches the house he gets a big tip.
An ancient puzzle