Why do my burgers puff up?

Chef's answer
The basic theory is that as meat cooks, the proteins contract so that the whole patty shrinks and puffs up in the middle. Making a kind of indentation or dimple in the middle counteracts that effect so you end up with a nice, even patty.
Frequently asked Questions 🎓
Eat them on their own with dairy, grain or nut milk, or add them to your favourite cereal. Puffed amaranth is also delicious in home-made mueslis and granola. For a quick and convenient snack sprinkle puffs over yoghurt and fresh berries. Or combine them with dried fruit, nuts and seeds in a scrumptious hikers' mix..
A few more cooking questions 📍