In spring and summer, a blue whale eats as much as four tonnes of food a day, about twice as much as a well-fed person eats in a year. It swims, mouth wide open, through the sea, sucking in thousands of litres of water which contains krill (tiny shrimp-like creatures). Instead of teeth, the whale has huge bony plates called baleen hanging down inside its mouth. They strain the water, leaving the krill.