Banana Hammer

States of Matter

• 2 Bananas
• balsa wood board
• a nail
• Liquid Nitrogen
• cryo gloves
• crucible tongs

• A banana is mostly made of water. Water is a liquid.
• The room temperature banana is too soft and mushy to hammer in a nail. Imagine trying to hammer in a nail with a water balloon.
• Freezing the water in the banana is one way to make it a better hammer.
• The water molecules in the banana start to slow down when you make them cold.
• They start holding onto each other more tightly, like the volunteers in the earlier demo.
• When the molecules get cold enough, the water in the banana becomes a solid (ice).
• With a frozen banana, the nail can easily be hammered because the solid water is much more rigid and tough than the liquid.

• DO NOT reach into the LN₂ with the cryo gloves. The LN₂ soaks into the gloves right away and traps it near your hand.
• Use the crucible tongs to fish the banana out of the LN₂ (it sinks to the bottom).
• Always use the cryo gloves when handling the frozen banana (or pieces of it, if it breaks up)
• The banana may break when hammering in the nail if it was left in the LN₂ too long. About 30 seconds to a minute is a good amount of time.
• Make sure the kids don’t grab pieces of banana. It stays cold for a long time, and will hurt their hands.

• Have fun with this demo! Its purpose is to show a liquid to solid transition, but it has always been funny to perform and watch. Ham it up.
• Have someone drop a banana into the LN₂ right at the beginning of the demo. That way it will be frozen by the time you need it, but not so frozen that it will totally shatter.
• Explain that you need to hammer a nail into the board.
• Unfortunately, the only thing you can use (or find, whatever) is a banana.
• See if the audience thinks it will work.
• Regardless of the response, tell them that YOU think it will work, and you’re going to do it.
• Hold onto the nail, just in case it’s near a previous hole. No matter what, make sure that the nail doesn’t go into the board for this part.
• Messy = funny. Make sure the banana goes all over the place, but try not to launch banana sectors at the audience.
• Comment on why that didn’t work so well, and have someone fish the banana out of the LN₂ carefully with the tongs.
• Explain that the banana is mostly made of water (if you want, you can ask them and see if they guess this).
• Water is a liquid.
• Liquids don’t make very good hammers.
• By this time (about a minute, but no less than 45 sec.) the banana should be solid enough to use as a hammer.
• Remove the banana from the dewar with the tongs and hold it with the glove.
• To make sure this goes smoothly, you may want to position the nail over a previously made hole.
• Hammer in the nail.
• Ta-Da! Act triumphant.

Click here for the Adobe PDF version of this demo (text only), for printing purposes.