Watching his four-year-old son tumble off a two-wheeler every time he slowed down inspired designer Scott Shim to invent a better training tool. At rest, the Shift bike's two back wheels tilt in at the top to form a stable, balance-assisting "A." When the rider leans forward-a natural tendency when pedaling faster-the weight shift activates a tension band that unites the bottom of the rear wheels into, effectively, a single wheel. As the rider slows, the wheels split again. The bike will be available this summer for about $100, minus what you'll save in Band-Aids.