Video narration
At the playground, if a runner looks still, how can you tell if they're moving? Use the fence to check. As the runner moves along, notice whether they stay beside the same part of it or pass it. If their position changes compared with the fence, they're in motion. The fence stays in place compared with the playground, so it is at rest relative to those surroundings. The key isn't whether something looks still; it's whether its position changes compared with what's around it. Now compare two runners covering the same distance. One gets there sooner. That runner takes less time, so they're moving faster; the other is slower. At the playground, compare with the fence: changing position means in motion; staying put means at rest relative to it. Over the same distance, less time means faster.