I was asked to show some science to my daughter’s preschool group and I thought that showing them how you can put a skewer through a balloon without popping it would be fun – and it was!
But how does it work? Why can you put a skewer through a balloon without popping it? Balloons are made from a material (often natural rubber, known as latex) that is able to stretch when a force is applied to it, and then go back to being unstretched when the force is removed. When a balloon is inflated, the balloon is forced to stretch by the pressure of the air inside the balloon. However, and this is the key fact that allows a skewer to go through a balloon without popping it, not all of the balloon stretches. These unstretched areas of the balloon can be pierced with a skewer and the balloon remains intact, just as it would if you were to push a skewer through the rubber of a balloon that had not been inflated.
A simple way to demonstrate this effect is to take a rubber band and stretch it, then cut the band while it is stretched. The rubber band pings back in the direction that it is being pulled. If you take a rubber band and cut it while it is not stretched the rubber band does not ping back.
Seems magical doesn’t it? That you can put a skewer through a balloon without popping it. The magic comes, as it often does, because our minds are deceiving us. We think the balloon is completely inflated but, in reality, it is not.
What parts of an inflated balloon are not stretched?
There are two regions on the surface of an inflated balloon that are not stretched (or not stretched too much):
- The neck of the balloon right next to the knot
- A small darker spot directly opposite the neck of the balloon
To push a skewer through a balloon without popping it, the skewer must enter and exit the balloon through these two regions. The location of these unstretched areas is very handy. It means a skewer can be “magically” pushed through one end of a balloon and out the other end of the balloon without popping it.
Why are these regions of the balloon not stretched?
The small dark spot, directly opposite the neck of the balloon, is there as a result of the way balloons are manufactured. This spot is slightly thicker than the rest of the balloon, as such it takes more force to stretch this spot than it takes to stretch the rest of the balloon.
The neck doesn’t inflate (stretch) unless the balloon is blown up so much that the pressure of the air inside the balloon starts to inflate the neck of the balloon. Balloons are not usually blown up to this extent as they are much more likely to pop and it also it leaves very little neck to tie a knot in the balloon.
Why are balloons stretchy?
Balloons are made from a material called an elastic polymer.
Elastic means it can stretch (or be compressed) and re-form back to the way it was before it was stretched (or compressed).
A polymer is a very long molecule. To visualise these polymers and how they make up the material for a balloon, I think a plate of spaghetti is a reasonable comparison. The molecules are lying around randomly, overlapping and squiggly (seems like a good descriptive word). The strands of polymer (spaghetti) are able to be pulled out straighter or made squiggly again – like the stretching and re-forming of the balloon. Strands of spaghetti are separate from each other, whereas the polymers in a balloon are linked together (“crosslinked”) – which helps to hold the balloon together (this crosslinking process is called vulcanization). To help visualise this crosslinking, and to carry on the spaghetti analogy, imagine some spaghetti sauce being added to the spaghetti and mixed in. The strands are now stuck together – but still flexible, and with regards to the balloon, can still be stretched.
As the skewer pushes through the balloon these polymers stretch until they reach their limit and break, forming small holes that the skewer passes through. The polymers then re-form back to their unstretched state and the balloon remains intact. Not only does the balloon remain intact but these unstretched polymers have been slightly compressed by the skewer passing through them so they push back against the skewer, forming a seal, which keeps the balloon inflated.
Why use lubricant when pushing a skewer through a balloon?
Adding a little bit of a lubricant, like soap or oil, to the skewer before pushing it through the balloon makes it easier to get the skewer through the balloon and makes it less likely to pop. The lubricant forms a thin film between the skewer and the balloon, reducing the friction as the two materials rub against each other. This reduction in friction means that less force is required to push the skewer through the balloon. Less force means it is more likely the balloon will not stretch too much as the skewer goes through the balloon and it will remain intact.
How to put a skewer through a balloon – step by step instructions
Warning! Adult supervision required – sharp objects
You will need:
- A balloon
- A skewer (wooden or metal or plastic, but ideally smooth)
- Lubricant, such as dishwashing liquid or vegetable oil.
- Inflate the balloon and tie a knot. Make sure the balloon isn’t blown up too much.
- Lubricate the skewer with a little dishwashing liquid or oil.
- Hold the balloon at the knotted end and carefully insert the skewer into the darker spot on the top of the balloon.
- Push the skewer towards the knot, then out of the balloon through the slacker material close to the knot.
Did it work? Or did it pop? Keep trying! Lubricate the skewer a bit more, see if that helps.
Other things to try
- How long does it take for the balloon to deflate with the skewer inserted?
- Does it work to put the skewer in the knot end first?
- What happens when you take out the skewer – does it deflate quickly or slowly?
- Can you do this with other sharp items, such as a knitting needle?
- Can you still put a skewer through a balloon without using the lubricant? Does this change the time it takes to deflate?