Not every whale has two blowholes. Whales that have teeth, like the killer whale, only have one -- they use it the same way, though. Toothed whales are generally smaller and faster than other whales, and typically spend shorter periods of time underwater before surfacing for air. This is why they generally only have a single blowhole -- they don't need to absorb as much oxygen at the surface, so their relatively smaller bodies don't need the extra air passageway.

One notable exception is the sperm whale, which is a toothed whale that can dive for more than an hour at a time. Though he only has one blowhole, it's still atypical compared to other toothed whales -- he is the only whale whose blowhole is positioned on the side of his head, rather than the top.