Why do bicycles stay up when moving?
For grown-ups: the science
Moving bicycles stay up more easily because their spinning wheels and gentle steering help keep the wheels under the rider.
A bicycle is dynamically stable at speed. Gyroscopic effects from spinning wheels contribute somewhat, but the main stabilizing mechanism is steering: when the bicycle leans, its front geometry and rider corrections steer into the lean, moving the contact points under the combined center of mass. At very low speeds, active rider control is especially important.
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Transcript
A bicycle rolls down the path with its wheels whirring. Why does it stay up when it is moving?
When a bike is still, it can tip over unless someone holds it. Gravity pulls it down toward the ground.
As the wheels spin, they help the bike resist quick tipping. A moving bike also steers itself a little when it leans.
When the bike leans, the front wheel turns toward the lean. That brings the wheels back under the rider.
So a moving bike can balance more easily because spinning wheels and steering help it stay upright.