In the morning, a hiker at the bottom of a mountain heads up the trail toward the top. At the same instant, another hiker at the top of the mountain heads down the same trail toward the bottom. Each hiker arrives at her destination by the end of the day. Fxplain why, no matter what happens to either hiker along the way (lunch breaks, turnarounds to fetch dropped pocket knives, or changes of pace), they meet on the trail sometime during the day.