Google this question and you’ll get lots of long answers describing complex chemical and neurological pathways involving enzymes, syn-propanethial-S-oxide and lachrymal glands. If you don’t have time to read all that – there’s a chemical in onions that gets into the air when the onion is cut and then it gets into your eyes. It irritates your eyes so they make tears to wash it away. However, that doesn’t really explain why the onion makes your eyes water, only how.

Why does the onion produce this noxious chemical? Is it a defence mechanism to stop animals from eating it? Maybe, but there are definitely some (other) animals that eat onions. So perhaps this is a mystery we’re still working on.

While we’re working on that mystery you could also look into the different ways to prevent onions from making your eyes water. People have suggested all sorts of ideas like putting the onion in the fridge, roasting it whole in the oven first, breathing through your nose with your tongue sticking out while cutting it, cutting it under water, cutting it near hot running water, whistling while you cut, wearing swimming goggles, and putting a piece of bread in your mouth. All of these are ideas your child could research to find out which are the most effective and practical.
​
So there you have it, playing with science can prevent tears! Which method did you find most effective?