Three college students are asleep, in three different rooms. Before turning in, each student has a smoke and tosses the cigarette butt into the wastebasket next to his or her bed. Each awakens to find a fire blazing in the basket. Each has a pitcher of water on the bedside table.
The engineering student tosses the contents of the pitcher into the basket and douses the fire. Satisfied that the fire is out, he rolls over and goes back to sleep.
The physics student takes a look at the basket, assesses the size of the fire, and quickly estimates how much water will be needed to douse it. She pours exactly that amount from the pitcher. Satisfied that the fire is indeed out, she rolls over and goes back to sleep.
Likewise, the math major wakes up, takes in the fire, and recognizes that there's enough water in the pitcher to put it out. Satisfied that a solution exists, he rolls over and goes back to sleep.