Imagine a steam locomotive waiting at the station. It’s ready to leave. The engineer opens up the throttle. What happens? At first the wheels just spin. The train doesn’t move one inch. Then, all of a sudden, the wheels stop and begin to grip the rails. Then, slowly at first, the engine begins to move. If you started running to catch it then, you’d probably miss it. That’s because by then it would have developed momentum.
Via Peter Verschuere