River Crossing

You need to transport a dog, a cat, and a mouse across a river. You have a single boat which can only carry yourself and one animal. The animals are all safe in your presence but if you leave the dog and cat together unsupervised, the dog will kill the cat; the cat, if left with the mouse, will eat it; the dog and mouse are ok together. How do you arrange the transport to get the animals across the river in safety?

Answer:

Carry the cat across the river first, then return and take the mouse across. Now return with the cat and transport the dog. Finally fetch the cat again. The mouse & dog make one crossing, the cat makes 3 and you make 7.