A trail, two days and a conclusion that seems impossible to guarantee until you find the right angle to look at it.
The monk and the mountain
Riddle statement
A monk climbs a mountain along a narrow path. It leaves at dawn and reaches the summit at dusk. Spend the night there.
The next day go down the same path. He also leaves at dawn and arrives at the foot at dusk.
He may have walked at different speeds in different sections, stopping at different places and resting at different times on each day.
Can you be sure that there is at least one point on the path that the monk passes through at exactly the same time on both days?