Why do onions and garlic smell so strong?

The smell comes from sulphur compounds. Onions and garlic take up sulphur from the soil and build it into specific molecules, which sit inert inside intact cells.

Cutting or crushing breaks the cells, enzymes meet those compounds and a fast reaction produces new volatile molecules. Volatile means they escape into the air and reach your nose easily — which is why a whole clove smells faint and a crushed one is pungent.

The smell stays on your hands because the compounds cling to skin and do not dissolve well in water, so soap alone struggles. Rubbing your hands on stainless steel helps — the metal surface binds the sulphur compounds and lifts them off.

Cooking changes the smell through the same chemistry. Heat deactivates those enzymes and breaks the pungent products down into milder, sweeter compounds. That is why raw garlic is sharp while roasted garlic tastes soft and almost sweet. It also explains a rule of thumb in the kitchen: add garlic late if you want its bite, early if you want its sweetness.