Question

Why does a mixtures of flour and water poured to a boiling water become sticky

Answer 1

Flour is mostly starch. When starch is heated in water it begins to gelatinize at about 60C. This means the starch granule absorbs large amounts of water as the molecules break free of their regular, even crystalline structure. The hard, solid granules expand into giant squishy sponges. So pouring flour into boiling water supplies the heat and water required to gelatinize most or all of the starch, resulting in a sticky paste.