An engineering and a mathematics professor go on a camping trip. And because they did not consume enough Ramen in grad school, they carry a couple of packets with them, so they can have some for breakfast as well as dinner. Every morning and evening one of them has to light a fire, go down the hill they are camping on to get water from a river, boil Ramen in the water, and then wash the Ramen pot back by the river.
The first night the mathematics prof does everything according to plan. He lights the fire, gets the water, cooks Ramen, then washes the pot and brings it back. The next evening, its the engineer's turn, and he decides to do his friend a favor. He covers the dying evening fire with brush so its easy to relight in the morning, and he brings back water in the pot after washing.
The mathematician wakes up promptly at six. He notices the water in the pot, and the dormant fire. He promptly throws out the water, and extinguishes the embers. Rubbing his palms together with satisfaction, he says: "Now that reduces it to a problem with a previously-known solution."