Hi there! A calorie is a tiny little unit that measures how much energy is in the food you eat. It's like a little piece of energy that your body can use to do all sorts of things, like run, jump, and play.
Now, when people say "a calorie is a calorie," what they mean is that it doesn't matter where the calorie comes from - whether it's from a carrot or a cookie - it still has the same amount of energy.
BUT, and this is a big but, not all calories are created equal. Some foods, like fruits and vegetables, also have important vitamins and nutrients that your body needs to stay healthy. Other foods, like cookies and soda, have a lot of calories but not much else. So while a calorie from a cookie might have the same amount of energy as a calorie from a carrot, the cookie isn't going to give your body what it needs to grow strong and healthy.
So, remember: a calorie is a tiny unit of energy, but some foods are better for you than others even if they have the same amount of calories.