Brain Teaser SMS

We live in a large tower block, 10 floors high. My 5 year old son goes to school each day using the lift from the 10th floor to the ground floor. However, when he returns in the evening, he uses the lift to get to the 5th floor and then uses the stairs for the remaining 5 floors. Why?

Answer: He cannot reach the required button, floor 5 is the highest he can reach at the moment!