Back when my parents were kids, they used to have their milk delivered in glass bottles. At the top of the bottle was a thick layer of cream which was caused by fat globules separating from the water in the milk. Homogenisation stopped all that. It’s a process where the milk is forced through cylindrical holes under immense pressure. This causes the fat to break up into smaller particles and when the milk is bottled the fat no longer separates. You’ll notice that the homogenised milk you buy from a store is all liquid and no cream at the top.