A mad scientist kidnapped you and led you into a big tower. Inside the tower were two pools of water. The first one was filled with salt-water to the brim, and in it were 50 piranhas. In the second pool, there was a lot more room but 2 hungry sharks. The mad scientist says that if you can stay in one of the pools of water for 20 minutes, he will let you out. Which one do you choose, and how do you survive?