Kids arm-wrestle all the time, but over the Internet? And there's nothing unusual about checking your weight, but in gallons? And all of this to illustrate lessons about networks and the search for life in outer space?

  That's the deal in a large addition to the New York Hall of Science that will open on Thanksgiving Day. As in the rest of the museum, it puts a premium on not just looking, but doing.

"You will not find a `Keep your hands off' sign anywhere in the building" Mayor Michael Bloomberg said Tuesday before officials opened the new 55,000-square-foot space to reporters and invited guests.

So in an exhibit about the prospect of life on Mars and elsewhere, visitors not only read about the importance of water to life, but they're invited to step on a scale that reveals how many gallons of water it would take to match their weight.

The Internet arm-wrestling is part of an exhibit about the nature of networks. It's set up so that a visitor can challenge a friend just 10 feet away, or face off against somebody at a participating science museum in another state. Information about the force the visitor applies to the aluminum hand and arm he grasps is transmitted over the Internet to a motor at the opponent's station, which then applies the same force to a mechanical arm there. Thanks to a video camera, participants can even see the opponent's face.

Nearby, a knot of mesmerized fifth-graders watched ants swarm over orange and apple slices and make their way back to the colony, which was presented as a living network. And other kids ran movies of crowds of people, fish, birds and other animals backward and forward, trying to see how the organization of the groups arises from behavior of individuals.

Another section of the museum's addition focuses on the science of sport — where kids checked their reaction times in mock racing cars by stomping on the gas pedal when they saw a "Go" signal. And on a surfboard, they discovered their balance was better if they spread their feet wider.