Gravitational waves are going through you all the time. They first make you a bit taller and thinner, then a bit shorter and fatter, and then leave you back where you started. The effect is very, very small indeed so we need very good instruments to detect them.
We use them to look at what is happening in the universe – a bit like a telescope but with gravitational waves it is more like listening to the universe rather than looking at it.
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