Why does flour make lumps in hot sauce?

Picture a pot of warm gravy on the stove, with steam curling up and a cosy smell drifting out. Have you ever found a round lump in it with your spoon? It feels soft on the outside and strange in the middle.

Those lumps are made of flour. So why does flour make lumps in hot sauce? Let's find out together.

Flour looks like dust, but it is full of starch, which is the part of flour that soaks up water. Each tiny speck of flour carries starch inside it.

A grown-up cook tips a spoonful of flour into the hot sauce to make it thick. The flour lands in small heaps, with thousands of specks pressed close together. At first, the sauce can touch only the outside of each heap.

The heat makes the starch on the outside of a heap drink up water very fast. Those specks swell and grow plump, like a dry sponge dropped in a bath. What do you think those plump, wet specks do next?

They turn soft and sticky, and they glue themselves to each other. Together they make a thin, gooey skin all around the heap. The starch has wrapped the heap up like a tiny parcel.

That sticky skin keeps the sauce from getting inside. So the flour in the middle stays dry and powdery. A heap with a gooey skin and a dusty middle is a lump!

When a grown-up cook tips flour into hot sauce, the heat makes the starch on the outside of each heap soak up water fast, swell and turn into a sticky skin. That skin keeps the sauce out, so dry flour stays trapped inside as a lump.

Down in the warm gravy, one little lump bobs along under the steam. Its gooey starch skin wraps it up snug. Deep inside, its dusty middle stays tucked away.