system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I get lightheaded when I stretch my body?

Even stretches where my head doesn't go below my waist, like having one leg up on a beam and leaning forward, I always get lightheaded and have to stop. Why is that?
gpt
Stretching tells your body to put more blood in a certain area. This decreases the pressure in other parts, like your head. Imagine your body is a long, semi-inflated balloon that's almost completely deflated at one end. When the balloon sends some of the air to the deflated portion, the rest of the balloon bets slightly less inflated as a result.

The lightheadedness is caused by a slight drop in blood pressure. Eventually, your brain gets used to it and you get back to normal thinking abilities.

IANADoctor, so there's probably a more in-depth explanation, but this is ELI5.