(answer)
Tom is a small, six-year-old boy. He can easily reach the lowest button when he wants to go down to the first floor. But when he wants to go up, and he is alone, he has a problem. He can't reach the top button, which is for the fifteenth floor. He can reach only as high as the tenth button. So, he has to get off at the tenth floor and walk up to the fifteenth floor.

However, if other people are in the elevator, he asks someone to press the top button for him.