Dolphins have these nifty little things called melons. No, not like the fruit XD. It's located in their head, and is used as underwater echolocation. It's like......ok, so you know how you can cup your hands and shout and it will sound louder? The point is, the melon basically concentrates their clicks and other noises and "shoots" the sound into the water. If the sound "wave" (eeeeyyyyyy) reaches something, a sound wave will bounce back to the dolphin, alerting it to general shape, pretty accurate location and if it sends it multiple clicks, where it's going. The melon itself is pretty much very specialized fat, maybe fatty tissue. Eh. I'm not an expert. But look it up its really cool!