 Hi, I'm Ashka and I'm a Yale 2020 intern at the San Francisco Public Library. Today we're going to be doing two really fun experiments and learning about surface tension and combustion. The materials you need for the first experiment are 1 or 2 tablespoons of black pepper, 1 cup of water, 1 tablespoon of dishwashing liquid and 1 small empty bowl. Feel free to pause the video now and gather the ingredients. First put some dishwashing liquid into a small bowl. Now add approximately 1 to 2 tablespoons of black pepper into the bowl of water and dip your finger into the bowl. Well, nothing special happens here. This is called a control. A control is something that is used as a standard for comparison for checking the results of an experiment. Next dip your finger into the dishwashing liquid and again dip it into the bowl of water. This time the black pepper instantly moves away from your finger. Let's find out why this happens. Surface tension is a force present within the surface layer of a liquid that causes the layer to behave as an elastic sheet. It's caused because of the attraction of the water molecules to each other. Water molecules have positive and negative charges. The positive charges are attracted to the negative charges. As you can see in the diagrams, the left diagram shows the forces between the water molecules. The water molecules in the body of the water are attracted to all the neighboring molecules equally. These are known as balanced forces. Because in this case the forces acting in the different directions cancel each other out and the molecule becomes free to move. However the molecules at the top of the water do not have any force pulling them up over the surface. The forces acting here are called unbalanced forces because the force acting downwards is higher than the force acting upwards and so it doesn't get cancelled out. This causes a skin to form on the surface of the water. Adding the soap causes the surface tension of the water to break. This makes the water spread out and flatten and while this happens the water takes the black pepper away from the source of the soap. Dishwashing soap acts as an emulsifier which means it allows different oils and water to mix. The water is then able to mix with the grease on the dishes better and so it helps to clean the grime. Now let's quickly move on to the second experiment. The materials you need for this experiment are one candle, one glass of water. You could add some food coloring or turmeric powder to the water to help make it more visible. An empty glass that is big enough to cover your candle and a plate. Feel free to pause the video here and gather your ingredients. First place the candle in the middle of the plate and pour some water into the plate. Now quickly cover the candle with a big glass. Why do you think the water moved up? Combustion is a chemical reaction that produces heat and light. Combustion needs oxygen. As the candle burns it uses up the oxygen in the glass. As the oxygen gets used up the water from the tray rushes into the glass and takes the place of the oxygen. Thank you so much for joining me today. Be sure to check out the rest of the STEM Challenges Health videos by SFPL librarians and Yale interns.