Presenter notes: Do not define anything yet. Send one student out of the classroom door, right round the building, and back in through the same door while the class waits. When they return, ask two questions in this order. Did you move? Obviously yes, and they may be out of breath. How far are you now from where you started? Silence, then somebody says nowhere. Tell them that both answers are correct and that physics has two separate words for them, and that by the end of the period they will never mix those words up again.