---
title: 'What happens to these balloons when you open the valve?'
source: 'https://youtube.com/watch?v=VKlulHwMxgU'
video_id: 'VKlulHwMxgU'
date: 2026-09-24
duration_sec: 93
channel: 'Veritasium'
---

# What happens to these balloons when you open the valve?

> Source: [What happens to these balloons when you open the valve?](https://youtube.com/watch?v=VKlulHwMxgU)

## Summary

This video demonstrates a classic physics experiment involving two balloons of different sizes connected by a valve. The creator predicts the air will flow from the smaller balloon to the larger one, but is surprised to find the opposite occurs. The explanation lies in the material properties of the rubber, where smaller balloons are stiffer and thus at higher pressure.

### Key Points

- **The Setup** [00:00] — The creator presents a big balloon and a small balloon connected by a valve, asking the audience to predict what will happen when the valve is opened.
- **Initial Predictions** [00:13] — The creator and another person speculate that the big balloon will get bigger, the small balloon will get smaller, or that they will equalize in size. The creator initially thinks nothing will happen due to atmospheric pressure.
- **The Surprising Result** [00:39] — When the valve is opened, the big balloon gets even bigger, which is counterintuitive. The creator is shocked and asks why this happened.
- **The Explanation** [00:52] — The smaller balloon is at a higher pressure because the polymer chains in the rubber are tangled and stiff. As the balloon inflates, these chains uncoil and relax, making the larger balloon have lower pressure.
- **Real-World Connection** [01:17] — The creator notes that this is why it's harder to blow up a balloon at first, but it becomes easier as it inflates, due to the relaxation of the polymer chains.

### Conclusion

The experiment demonstrates a counterintuitive principle of physics and material science, where the larger balloon expands because it is at a lower pressure than the smaller one. This is due to the elastic properties of the rubber, which are not immediately obvious.

## Transcript

I've got a big balloon and a small balloon separated by a valve. What happens when I open it? How is that air going to flow? What's going to happen to the big balloon? What's going to happen to the small balloon? Hmmmmmm. Um, um... I think that maybe slope sizes.
I think nothing's going to happen. So with the atmospheric pressure, I think nothing will happen. Nothing's going to happen? This one's going to get bigger because I feel like this one's already like too full with air. Maybe this one will pull air inside that one. Probably the air is going to push from this one to the smaller one
because there's more pressure coming from this side. from this side. I'd say they become equal sized. They will stay the same. Let's test it out.
The big balloon gets bigger, which is super counterintuitive. What the ? What? But I thought,
but why? Why? Why? So why did the big balloon get even bigger? The smaller balloon was actually at a higher pressure. That's because when a balloon is smaller, the polymer chains inside the rubber are tangled up tight. And that makes a smaller balloon much stiffer.
But as it inflates, those polymer chains uncoil and they relax, which means that the larger balloon is actually at a lower pressure. It's pretty counterintuitive, but you might have noticed it if you've ever,
you know, blown up a balloon and noticed it at the beginning is a lot harder. It's a lot harder and then it becomes a bit more easier. Is it getting easier? Yeah, it is. Yeah. Crazy. Yeah, there you go. I wouldn't think about it. Awesome.
Thank you for talking everything.