 Welcome to San Francisco Public Library's STEM Challenge Yourself. I'm librarian Joseph. You might remember me from previous challenges as presenter and host. For this week, I'm very excited to introduce four amazing STEM challenges produced by Yale 2020 interns. For today's challenge, our intern Ashka will introduce us to the fascinating world of gravity. Are you ready? Ashka, please take over. Hi, I'm Ashka and I'm a Yale 2020 intern at the San Francisco Public Library. Today, we're going to be discussing the concept of density through two really interesting experiments. First, let's come up with a hypothesis. A hypothesis is an educated guess or a guess you make based on the information you already know. What do you think would happen if you put an unpeeled grapefruit in the water? Do you think it'll float or will it sink? What do you think will happen if you peel the grapefruit? The materials you require for the first experiment are one grapefruit and one large container with water in it. First, place the grapefruit gently on the surface of the water. It floats. Now peel off all the skin except for the part on the top of the grapefruit. It still floats. Lastly, peel off all the skin and again place the grapefruit in the water. It sinks this time. Was your hypothesis right? If yes, that's amazing. If not, don't worry, that's completely okay too. Now let's find out the science behind what happened. Buoyancy is the tendency of an object to float or sink in water or any other fluid. When the grapefruit is placed in the water, there are two forces working in opposite directions. The gravity pulls the grapefruit down with a force which is equal to the weight of the object. Buoyant force pushes the grapefruit upwards with the same force as the weight of the water that the grapefruit moves. So, if the grapefruit moves water which is equal to or more than its weight, it will be able to float. Density is the amount of mass per unit volume. The unpeeled grapefruit floats because the buoyant is very porous and filled with lots of tiny pockets of air. This makes the grapefruit less dense and displaces more water which makes the unpeeled grapefruit float. When you remove the peel, the grapefruit doesn't displace enough water and it becomes more dense and sinks. Now, for experiment two, you need two eggs, two glasses of water, three tablespoons of salt and a spoon to stir the solution. What do you think will happen when you put an egg in the water? Why do you think that will happen? First, carefully put one egg into one of the glasses of water. Oh no, it sank. The egg sinks when you put it into a glass of water because the density of the egg is higher than the density of the water. That's okay, I have an idea. Put three tablespoons of salt into the other glass of water. Stir the solution until the salt is dissolved. Now, gently put the other egg into the water. It floats. If you add enough salt to the water, the mass of the water increases without the volume changing too much. This causes the density of the water to increase. Now, the egg would be less dense to the water so it would float back up to the surface. Water is denser than fresh water. Some things float more easily in the ocean and extremely salty bodies of water like the Dead Sea. You can experiment with different amounts of salt and different objects to see what happens. You could also repeat this experiment with some sugar. Thank you so much for joining me today. Be sure to check out the rest of the STEM Challenge Yourself videos by SFBI librarians and Yale interns.