Archimedes went to sleep beside a big rock.

He wanted to get up at 8 AM, but the alarm clock was yet to be invented! He decided to sleep at the spot where the rock's shadow should end when it's 8 AM so as to be awakened by the direct sunlight.

Archimedes knew that at 8 AM, the sunlight reaches the ground at an angle of 43°. The rock beside which he slept was 7 meters tall.