ELI5: What causes kidney stones?

Kidney stones are caused when tiny bits of waste in your pee stick together and turn into hard little pebbles because there is not enough water to wash them away.

Imagine making a glass of chocolate milk. If you stir one spoonful of chocolate powder into a tall glass of milk, the powder disappears completely. But if you put ten huge scoops of powder into just a tiny splash of milk, the powder cannot dissolve. Instead, it turns into thick, crunchy clumps at the bottom of the glass.

Inside your body, your kidneys act like magical trash filters. They clean your blood and send the leftover waste out of your body as pee. Usually, when you drink plenty of water, this waste stays completely melted and flows out smoothly without you ever feeling it.

However, stones start to grow when everyday habits upset this balance:

• Not drinking enough water: When you run around on a hot day and forget to drink fluids, your body runs low on water. Your pee becomes dark and crowded with waste, making it very easy for little crystals to bump into each other and stick together.
• Eating too much salty junk food: Eating lots of very salty chips or processed snacks pushes extra calcium into your pee. Calcium is normally great for your bones, but too much of it floating in your kidneys can glue other waste particles together.
• Drinking too many sugary sodas: Certain sweet drinks contain ingredients that make your pee more sour, which helps these crystals clump together faster.