 Hi, I'm Ashka and I'm a Yale 2020 intern at the San Francisco Public Library. Today we'll be learning the science behind making cottage cheese. Please note that young children may require adult supervision in this experiment. The materials you need for this experiment are half a gallon of milk, a one large strainer, two cooking pots, one bowl, three to four tablespoons of vinegar and something to stir the mixture with, along with an extra spoon. Feel free to pause the video here and gather your ingredients. Firstly, pour half a gallon of milk into a cooking pot. Turn on the heat and let the milk heat until it's almost boiling. Add around three to four tablespoons of vinegar slowly to the milk and continuously stir the mixture. Stop adding the vinegar when the milk begins to curdle as shown. You may require more or less vinegar depending on the milk that you use. Stir the milk for a few more minutes until it stops curdling. Why do you think the milk curdles when you put the vinegar in it? Separate the liquid part from the solid using a strainer as shown. Gently press on the solid to make sure that all the liquid is drained out. But be sure to not press too hard because otherwise the cheese may not be as soft as you want it to be. Now transfer the cottage cheese into another bowl or plate and enjoy. Delicious! Now let's find out the science behind what happened. Curds are thickened or solid parts that form in sour milk and are the beginning of cheese. Whey is the watery part of milk that separates after the milk has soured and thickened. Cacine is the main protein present in milk and cheese. After you add the hot milk into the vinegar small white chunks or curds become visible in the mixture. This is because when you add an acid like vinegar to the milk it changes the pH of the milk and makes the cacine molecules unfold and reorganize into a long chain which curdles the milk. 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.