Walt is taking an overnight backpacking trip on the Appalachian Trail in Georgia. He picks up his backpack from the ground, puts it on his back, and walks five miles before he stops and puts the pack down near a stream. He stands near the stream and fishes for dinner, but doesn't catch anything. “I guess I’ll have soup for dinner!” he thinks, and lights a fire to heat up his soup. At what point in the day did Walt do the most work, according to the scientific definition of the word?