HERE'S SOMETHING intriguing to promise a kid: "Today we're gonna go see a turtle that's so tough it's not even afraid of sharks. Then we'll look at a gang of piranhas with razor-sharp teeth. And if you want to, you can play with a real live horseshoe crab."

Uh-oh, you think, a trek to Baltimore?

Not at all. Washington has its own aquarium, and it's a relative cinch to get to. Housed in the basement of the Commerce Department, Washington's National Aquarium is smaller, less advertised and far less grand than the one at the Inner Harbor. But it does have some dandy features.

One is a low admission fee: $2 for adults and 75 cents for seniors and children 3 to 12. Another is a "touch tank" exhibit where kids can pick up and examine horseshoe crabs and other harmless beach dwellers. A third feature is the variety of animals to look at -- more than 250 species in all.

The exhibit that kids always home in on, of course, is the shark tank. Saturday that should be a particularly hot spot since it's Shark Day at the aquarium. Thankfully, the day's highlight -- a 2 p.m. shark feeding -- is not a one-day-only offer. In fact, the public can watch shark feedings every Saturday of the year, as well as on Mondays and Wednesdays.

The three nurse sharks and three lemon sharks presently at the National Aquarium are all babies between 20 and 48 inches long. When one grows larger than four feet, it begins to have trouble negotiating the turns in the 6,000-gallon tank, and curator Michael Bailey must find it a new home or release it into the ocean. Then he acquires a new baby shark to replace it.

Make no mistake about it, though: These little rascals are chompers.

"They could hurt you some," Bailey confirms proudly, peering into the tank. "Certainly any bite by a shark involves a good deal of tissue damage."

Cruising around among the sharks are three sea turtles that seem completely at ease among their tankmates. Bailey explains that the turtles have little fear of shark attacks because they, too, are equipped with powerful jaws, sharp teeth and assertive natures -- not to mention a strong shell.

"If it's feeding time and they're hungrier than the sharks," he says, "they'll push the sharks right out of the way."

Aiming to prove the point, Bailey hurries over to a doorway and disappears. A minute later, three squid chunks splash into the tank and begin drifting toward the bottom. The largest turtle, 21-year-old Spunky, swims to the nearest chunk and devours it in three distinct chomps. He consumes several more pieces with similar dispatch. The sharks, clearly not hungry enough to put up a fight, ignore the snacks.

A while later, Bailey watches as a single squid chunk gets tossed into the piranha tank by an aquarium volunteer. The 28 red-bellied piranhas are round and thin, and not much bigger than a person's hand. They don't look a bit ferocious -- in fact they appear benign -- until one of them darts up and seizes the squid, whereupon the others seem to simultaneously have the same thought: "MY LUNCH!"

They all zoom up to the first fish and start ripping off pieces of its squid chunk. Smaller fights break out over the ripped-off morsels. Within five seconds, the first course is history. Then a second squid chunk splashes into the tank and, like tireless actors, the piranhas take it from the top.

It's no wonder public piranha feedings (every Tuesday, Thursday and Sunday at 2 p.m.) are nearly as popular as shark feedings.

Ferocious predators, alas, are somewhat less compelling when seen between meals. But kids are apt to enjoy studying the dozens of other creatures in the aquarium. The lookdown fish, so named for its permanent facial expression, is so thin that it appears to be a swimming slice of glass. The alligator snapper turtle has a worm-like appendage on its tongue; small fish approach this bait hungrily and get snapped up by the turtle's jaws.

Sea anemones resemble beautiful plants, but they sting and consume small prey that stray too close. Gitcha, Getcha and Gotcha, alligator siblings caught by Bailey in Florida, like to lie beside their glass wall and stare back at inquisitive visitors. And horseshoe crabs, whose thick shells and skinny tails make them look so sinister at the beach, turn out to be docile and pettable when kids approach them at the touch tank.

All in all, it's a deeply enjoyable experience.

SHARK DAY -- 12 to 5 Saturday at the National Aquarium, 14th Street and Constitution Avenue NW. 377-2826. First 500 families receive Shark Bites candy; other door prizes include inflatable sharks, buttons and National Aquarium Society memberships. From 12 to 2, face-painters will give kids a free rubber shark nose. Behind-the-scenes shark tours will be given at 1, 3 and 4 p.m. to the first 15 people entering the aquarium at 12, 2 and 3, respectively. At 2, shark specialist Michael Bailey will feed the sharks and answer questions. A costumed "land shark" will pose with children from 3 to 5. Tapes of the Discovery Channel's shark program will run continuously in the aquarium theater. The aquarium is open daily 9 to 5. Metro: Federal Triangle.

Kevin McManus last wrote for Weekend about the Broadcast Factory.