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

Source: https://www.youtube.com/watch?v=VKlulHwMxgU
Recap page: https://rapidrecap.app/video/VKlulHwMxgU
Generated: 2026-10-01T18:03:53.63+00:00

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## The Gist

When you open a valve between a large balloon and a small balloon, air flows from the small balloon into the large balloon, making the large balloon even bigger. This happens because a smaller balloon is actually at a higher internal pressure due to the tight, uncoiled polymer chains in the stretched rubber.

## Quick Overview

Air flows from the smaller balloon into the larger balloon when a connecting valve is opened, making the big balloon grow even larger because smaller balloons possess higher internal pressure. Veritasium demonstrates this counterintuitive physics phenomenon to random people on the street before explaining the mechanics of rubber elasticity. The higher stiffness of a small balloon stems from tangled polymer chains that require more force to stretch initially, but they relax as inflation increases.

**Key Points:**
- A valve connects a large balloon and a small balloon filled with air.
- Multiple pedestrians guess that nothing will happen, that the sizes will swap, or that the larger balloon will deflate into the smaller one.
- When the valve opens, the big balloon gets visibly bigger while the small balloon shrinks entirely.
- The smaller balloon holds a higher internal pressure than the larger balloon.
- Polymer chains inside the rubber are tangled up tight when a balloon is small, making the material much stiffer.
- As the balloon inflates, those polymer chains uncoil and relax, which results in a lower internal pressure for the larger balloon.
- Blowing up a balloon is hardest at the very beginning and becomes noticeably easier as it expands.

![Screenshot at 0:38: The big balloon unexpectedly inflates further as the valve opens and air flows out of the smaller balloon.](https://ss.rapidrecap.app/screens/VKlulHwMxgU/00-00-38.jpg)

**Context:** Veritasium host Derek Muller tests physical misconceptions on the street by asking pedestrians to predict counterintuitive mechanical outcomes. Fluid dynamics and material science often produce results that contradict everyday intuition, such as pressure gradients in elastic membranes.

## Detailed Analysis

The video investigates the pressure dynamics of connected balloons of different sizes. Derek Muller approaches people on the street, asks them to predict what will happen when a valve connecting a large and a small balloon is opened, and records their incorrect hypotheses. When the valve is actually opened, the large balloon grows larger at the expense of the small balloon. Derek explains that this occurs because small balloons are at a higher internal pressure due to tightly tangled polymer chains in the rubber, which relax and uncoil as inflation increases, lowering the internal pressure of the larger balloon.

### Street Predictions

- Passersby offer various incorrect guesses about the outcome, including that the balloons will swap sizes or that nothing will happen.
- One person correctly notes that the smaller balloon might have more pressure, but most rely on faulty intuition.

![Screenshot at 0:11: Pedestrians guess what will happen when the valve opens between the two balloons.](https://ss.rapidrecap.app/screens/VKlulHwMxgU/00-00-11.jpg)

### The Counterintuitive Result

- Opening the valve causes the big balloon to grow even bigger while the small balloon collapses.
- The participants express total shock and confusion at the unexpected direction of airflow.

![Screenshot at 0:39: The big balloon gets bigger, shocking the participants.](https://ss.rapidrecap.app/screens/VKlulHwMxgU/00-00-39.jpg)

### The Physics Explanation

- The smaller balloon maintains a higher internal pressure because its rubber polymer chains are tightly tangled.
- As a balloon inflates, the polymer chains uncoil and relax, which creates a lower internal pressure in the larger balloon.
- This phenomenon explains why blowing up a balloon is physically hardest at the very beginning of the inflation process.

![Screenshot at 0:56: Explanation of how polymer chains and rubber elasticity dictate internal pressure.](https://ss.rapidrecap.app/screens/VKlulHwMxgU/00-00-56.jpg)