Why does cereal sink after it has been in milk?

* Absorption: The milk gets absorbed into the cereal, making it heavier. The cereal itself doesn't change much in size, but its density increases.

* Starch gelatinization: The starch in cereal grains absorbs water and swells. This process, called gelatinization, makes the cereal denser and more likely to sink.

* Sugar dissolution: Some cereals have added sugar. This sugar dissolves in the milk, leaving behind a denser, heavier cereal.

Why does it float at first?

When you first put cereal in milk, it floats because:

* Air trapped in the cereal: Most cereals have air pockets trapped in the structure of the flakes or other shapes. This air makes them less dense than the milk, so they float.

* Surface tension: The surface tension of the milk helps keep the cereal afloat.

As the cereal sits in the milk, the air escapes, the starch gelatinizes, and the sugar dissolves. This leads to a heavier cereal that's no longer buoyant enough to stay afloat.