Here's why:

* Tension is a force that pulls on an object, stretching it.

* In a beam, this tension force is generated when the beam is subjected to bending forces.

* The bending force causes the top of the beam to be in compression (pushed together) and the bottom of the beam to be in tension (pulled apart).

Think of it like this: when you bend a ruler, the top side of the ruler is squeezed (compression), and the bottom side of the ruler is stretched (tension).

Let me know if you'd like more details on beam bending!