 2020 intern at the San Francisco Public Library. Today we are going to make a DIY balloon powered hovercraft. Let's get started. For this craft you're going to need a CD, a balloon, a bottle cap, a glue gun, and a pair of scissors. You can pause the video right now to go gather your materials. First take the tip with the glue gun and gently press it onto the middle of the bottle cap. Give it a couple seconds and this should mount a hole in it. You can use scissors to make the hole bigger if you need to, but if you have a smaller and thinner bottle cap you can simply poke the hole with your scissors. Another alternative to this would be using a needle and poking several holes onto the bottle cap. This way you can control the airflow by choosing how many holes to poke. Next you're going to take the bottle cap and glue it onto the center of the CD using the glue gun. Make sure to be careful with the glue gun. Never touch the tip or the top in general because it is very hot and it will burn your finger. You don't have to put too much glue and it should still stick fairly well. Be sure to turn off your glue gun or unplug it because you'll no longer need it in the next steps. Now you're going to take your balloon, blow it up, and twist the end of it. Twisting the end will prevent the air from releasing too early. Then you're going to wrap the end of the balloon around your bottle cap. This may take a few tries because the balloon does sometimes slip, but after you're done it should look something like this. You might want to hold the balloon where it wraps around the bottle cap or where it twists so no air releases involuntarily. This is what it would look like when it slips, but don't get too frustrated because you'll get it eventually. Now release it on the table and have fun with it. You can even test it out on different surfaces to see how it would work. So what's the science behind this? Normally when you push the CD on the table it barely slides. This is because of friction. Friction is the resistance force that occurs when two surfaces slide against each other. As you can see in the picture the blue arrow shows the direction you are pushing the object and the red arrow shows the friction force which is in the opposite direction of the blue arrow. In this case when you slide the regular CD on the table the friction force goes against the way you slide it causing it to stop. Sliding on a rougher surface like a carpet would also increase the friction force causing it to stop faster. So how does attaching the balloon on the CD make it glide smoother? When you first pull up the balloon you're filling it up with air molecules also shown as the blue specks in the picture. After you attach it to the CD the air molecules escape the balloon through the hole of the bottle cap and go in between the CD and the table. Now there's a layer of air molecules in between even if it's too small for the human eyes to see. The layer of air molecules reduces the friction between the table and CD allowing it to glide smoothly. As the air from the balloon runs out and all the air molecules are released the air cushion is no longer there and the CD slows down until it comes to a complete stop. This is also the same concept as air hockey which you have probably either played or seen at the arcade before. However in air hockey the air continuously comes out from the table so the puck doesn't stop gliding mid game. The air released from the table reduces the friction between the puck and the table. Be sure to check out the rest of the STEM challenge yourself videos by SFPL librarians and Yale interns. Thanks for watching!