There was a smart philosopher from Greece named Thales. He lived a long time ago in the 6th century BC, in a place that is now called Türkiye. Thales visited Egypt and was amazed by the huge size of the pyramid built by Khufu.

Thales wanted to find out how tall the pyramid was. One sunny day, he put a stick in the ground near the pyramid. He watched as the shadows of the pyramid and the stick got longer throughout the day. He patiently waited until the shadow of the pyramid was the same length as the stick. Then he said, "The length of the pyramid's shadow right now is equal to the height of the pyramid!"

Thales realized that the shapes formed by the stick and its shadow, and the pyramid and its shadow, were similar. This means that they had the same shape, just different sizes. By using this idea, Thales was able to calculate the height of the pyramid.