Sieving

Malli and Valli are about to take a train to Ahmedabad. Valli asks her Mami to make meethi puri (a sweet bread of wheat flour) for the journey and offers to knead the flour. Mami says: first, remove the bran that may be in the flour. They use a sieve.

A sieve has many small holes. When flour is shaken on it, the fine flour passes through the holes. The bigger bits, like bran and small stones, cannot fit through, so they stay on the sieve. This is sieving.

Sieving works when the parts of a solid mixture are different sizes: small ones fall through, big ones stay on top. If both parts were the same size, a sieve could not separate them.

Sieves are used for big jobs too. At building sites, workers use large sieves to separate pebbles and stones from sand. The fine sand passes through; the pebbles and stones stay on the sieve.