Why Is Milk White?

'But Why' heads to the farm to answer a whole herd of animal questions: How do cows make milk? Why do cows moo? Why do some animals eat grass? Why do pigs have curly tails? Why do pigs have more teats than cows? Why do eggs in the fridge not hatch? How do chicks grow in their eggs? Why do roosters crow? Why do horses have hooves? Why do horses stand up when they sleep? Why are some fences electric?