Why onions make you cry
An onion stores sulphur compounds in its cells. Your knife breaks them open, a new gas forms, and your eyes flush it out with tears.
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Transcript
The onion stores sulfur
An onion is mostly water and carbohydrates, but it also holds sulfur compounds locked inside its cells.
These sulfur molecules have no smell and cause no tears while the onion is whole.
Your knife breaks the cells
When you cut an onion, you rupture thousands of cell walls in seconds.
Two chemicals that were kept separate now mix together: an enzyme and a sulfur compound.
They react instantly and create a new gas called syn-propanethial-S-oxide.
The gas floats upward
Syn-propanethial-S-oxide is volatile—it evaporates into the air above your cutting board.
This gas rises and drifts toward your face within about thirty seconds of the first cut.
Your eyes detect an irritant
The gas reaches your eyes and touches the cornea, the clear front layer.
Nerve endings in your eye sense it as a threat, the same way they sense smoke or pepper spray.
Your body does not know the gas is harmless.
Tears wash the irritant away
Your tear glands flood your eyes with salt water to dilute and flush out the gas.
The tears roll down your cheeks—not because you are sad, but because your eye is defending itself.
Onions weaponize chemistry to protect their offspring seeds from being eaten, and your tears are proof it works.
Every sentence above was written before a single picture was drawn, then each object it names was painted for this video alone.
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