Why does milk turn into yogurt?

Milk becomes yogurt when helpful bacteria eat lactose, the sugar in milk, and make lactic acid. That acid changes the milk proteins so they cling together, turning a pourable liquid into a soft, spoonable food with a tangy taste.

The remarkable detail is that yogurt is still busy with life. Different bacterial teams create different textures and levels of sourness, and straining removes watery whey to make a thicker bowl, the way Greek yogurt becomes dense and creamy.