Dolphin vs. Porpoise

We see many sea creatures while sailing the SF Bay.  One that we see more and more of are porpoises.  My crew is always asking me the difference between them and dolphins.  Here is a short answer:
People use the terms dolphins, porpoises, and whales to describe marine mammals belonging to the order Cetacea (from the Greek work ketos, “large sea creature”), and often use them interchangeably. The orca, or killer whale, for example, is actually the largest member of the dolphin family.
Dolphins are by far more prevalent than porpoises. Most scientists agree that there are 32 dolphin species (plus five closely related species of river dolphin) and only six porpoise species.
So what’s the difference? It essentially comes down to their faces (who can forget Flipper’s famous “grin”?), their fins, and their figures. Dolphins tend to have prominent, elongated “beaks” and cone-shaped teeth, while porpoises have smaller mouths and spade-shaped teeth. The dolphin’s hooked or curved dorsal fin (the one in the middle of the animal’s back) also differs from the porpoise’s triangular dorsal fin. Generally speaking, dolphin bodies are leaner, and porpoises’ are portly.
Dolphins are also more talkative than porpoises. Dolphins make whistling sounds through their blowholes to communicate with one another underwater. Scientists are pretty sure that porpoises do not do this, and some think this may be due to structural differences in the porpoise’s blowhole.
Dolphins and porpoises have many similarities, one of which is their extreme intelligence. Both have large, complex brains and a structure in their foreheads, called the melon, with which they generate sonar (sound waves) to navigate their underwater world.  Dolphins are one of the few species that have sex for fun!  Can you name the other two?
It is likely that more (or fewer) differences between dolphins and porpoises will be revealed as researchers continue to investigate these intriguing sentinels of the sea.