We actually lose height on a daily basis. Measure yourself in the morning and again at bedtime, and you'll see that you've lost about 1cm over the day. That's because gravity pulls us down to the surface of the Earth, compressing our bodies. The flexible discs between the bony bits in our spines, which are mostly water, get squashed until we lie down, then they fill out again. Astronauts have been shown to grow up to 7cm when they go into space, as they don't have gravity pulling on them. It's actually quite painful.
As we get older, we get permanent height loss. We produce less of the hormones that help us to repair or replace cells. This means our bones become weaker. Our vertebrae – the bones that help keep us upright – get compressed and rubbed all the time, so we might lose some of the actual bone. Our muscles and ligaments also get weaker, which adds to the effect.
You don't start shrinking when you're quite old; it happens a long time before that. On average, humans lose 1cm of height every decade after the age of 40.
• This is the final Ask A Grown-Up. A big thank you to our younger readers for all their questions over the last two years.