This is an excellent question. In fact, it is such a good question that scientists themselves are still trying to answer it. And whales and marine mammals are not the only ones who breach. Some rays and sharks also breach. It is not known exactly why they breach, but there are a number of different hypotheses. One hypothesis is that they are trying to shake parasites off of their skin.

The impact of the animal hitting the water may have the same effect for the marine animal (whale or ray, for example) as a dog rubbing itself against a tree to try to scratch fleas off of its back.

Another hypothesis is that breaching allows them to communicate with each other; it is possible that the sound the animal makes as it breaches may signal to something to others. In fact, in some cases humpback whales may breach more frequently in rough seas, when their songs will be harder for other whales to hear. They may breach simply to have a look around at what is going on above the water (if, for example, they hear something like boat but can't see it). Finally, the breach may be the end of some complicated underwater behavior that we can't see from the surface. And of course it may just be fun...

Nobody is sure why sea mammals breach, but there are is evidence to support a number of good theories. It is thought that breaching helps remove barnacles and lice from their skin. Baby whales that lost their mothers have been seen to breach repeatedly--probably to send a visible and audible signal to the mother. (Breaching is very loud, and sound travels very well in water.) Adults sometimes appear to breach to scare off an intruder, and some people think they may simply do it because they enjoy it.

That's a good question! Because we can't ask them, we'll never really know. Scientists have many hypotheses (or guesses) as to why whales breach. One idea is that they are trying to get rid of parasites or other organisms that grow on the outside of their skin (eg barnacles). Perhaps the parasites or barnacles cause their skin to itch, and breaching is a way of scratching themselves when they're far from land and the bottom is too deep. There are killer whales in Canada that rub themselves on smooth stones on the bottom of the ocean, so breaching might be serving the same purpose.

Another guess as to why whales breach is that it's fun. Dolphins definitely like to play, so there's no reason why whales wouldn't like to play as well. During breading season, sometimes male humpback whales accompany female humpback whales and their calves as they swim long distances back to their feeding grounds. Sometimes unattached males will try to mate with the female humpbacks, and breaching is a way of displaying aggression and warding them off.

Lastly, breaching may be a way to communicate ("Here I am! Over here!"), by generating a large splash and a loud noise.

Scientists aren't entirely sure why whales and other sea mammals breach. One idea is that they do this to knock little animals and parasites off their bodies. Another popular theory says that they might breach simply to scratch an itch-- they don't have arms and fingers to scratch with, so maybe by jumping into the air and hitting the water they can slap the itchy spot for some relief! Of course, it's also possible that they breach for the same reasons people do a lot of interesting and weird things: because it's fun!