Why do onions make us cry?

Someone in the kitchen starts cutting an onion, and soon everybody’s eyes go watery. Nobody is crying about anything — so why the tears?

Onions keep a secret inside

An onion looks calm on the outside, but inside it is full of tiny juices. The onion keeps these juices locked away to protect itself from hungry bugs and animals.

When a knife cuts the onion, it breaks open all those little parts. The juices mix together and make a special stinging gas. You can’t see it, but it floats up out of the onion and into the air.

The gas reaches your eyes

The gas drifts upward and bumps right into your eyes. It feels prickly and sharp, a bit like a tiny sting.

Your eyes do not like that one bit. So they jump to the rescue with their best clean-up trick.

Tears wash the sting away

Your eyes make tears to wash the stinging gas away and keep your eyes safe and comfy. The tears flood out and carry the sting with them. That’s why you cry over an onion even when you feel perfectly happy!

A clever trick: chop the onion near cold water, or cool it in the fridge first, and far less stinging gas floats up.

Wonder fact: Your eyes make tears for onions, dust, sneezes and sadness — tears are your body’s all-purpose way of washing and protecting your eyes!