Humans have a love-hate history with milk.
For thousands and thousands of years, adults couldn't digest dairy products without an upset stomach and a trip to the bathroom.
And then one day, poof!
A few changes in our DNA gave about a third of the world's population – mostly Europeans — the ability to knock back cheese, pizza and chocolate ice cream without a care in the world.
But why? Why did this ability to digest lactose suddenly crop up in our European ancestors about 10,000 years ago? That's been a big mystery for scientists.