Answer:
The hole on the top of the dolphin is the blowhole.
Dolphins breathe by getting air through the blowhole, which can empty and refill the lungs of a dolphin in one-fifth of a second.
The blow is explosive and the air reach speeds of up to one hundred miles per hour.
A nerve mechanism alert the dolphin when the blowhole is out of the water, so it can blow out the air and breathe in again.
Strong muscles close the blowhole when dolphins dive into the water to avoid drowning.