Sieves let some things through — Engineering, 4–6 years
A sieve or filter uses holes or a barrier to let smaller or wanted things through while holding back others.
A barrier can sort
A sieve has holes, while a filter may have tiny spaces or a special material. Small pieces, water or air can pass through, but larger pieces or some dirt stay behind. The size of the holes and the thing being separated decide what gets through.
Why separate things?
Mixed things are not always useful together: stones can spoil flour, leaves can block water, and sand can make water cloudy. People needed a way to keep one part while letting another part pass. Sifting and filtering solve that problem without picking every piece out by hand.
Separating pasta water
Put three cooked pasta pieces and a spoon of water into a colander over a bowl. Tip the mixture slowly. The water runs through the holes into the bowl, while the pasta stays in the colander because it is wider than the holes. The colander has separated two things by size.
A filter cannot catch everything
It is easy to think that clear water must be completely clean, or that any filter catches every kind of dirt. This feels sensible because the dirt we can see may disappear. But some tiny particles or dissolved substances can pass through. A filter only removes what its holes or material can hold back.
Filters in everyday tools
Colanders separate food from water, tea strainers hold back leaves, and air filters catch some dust. Drains often use a grid so water can leave while larger rubbish stays out of the pipe. These tools work only for the particles their holes or material are made to stop.
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