Catching actual molecules
How Does Smelling Work?
Smelling means molecules from an object physically reaching receptors in your nose.
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Step 1 of 5
Smells are real molecules travelling.
Bits of the object genuinely break away and float to you. Smelling bread means bread molecules are physically inside your nose.
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Step 2 of 5
Receptors recognise molecule shapes.
Not one detector, but four hundred, each waiting for a shape it knows. High in your nose sit around 400 types of receptor, each responding to particular molecular shapes. A molecule fits, and the cell fires.
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Step 3 of 5
Combinations give the variety.
One smell activates a particular set of receptors. With 400 types, the number of distinguishable combinations runs into the trillions.
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Step 4 of 5
The signal goes almost straight to memory.
Smell connects directly to the brain's memory and emotion regions, bypassing the relay other senses use. That is why a smell can floor you with a memory.
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Step 5 of 5
Most of taste is really smell.
Your tongue detects only a handful of basic tastes. Flavour is mostly odour reaching your nose from inside your mouth — which is why food is dull with a cold.
Dogs have hundreds of millions of receptors to our few million, and far more brain devoted to processing them.
The short version
Around 400 receptor types, each waiting for a shape, combine into trillions of distinguishable smells. The signal skips the relay other senses use and goes straight to memory and emotion — and most of what you call taste is this.
Try it yourself
Hold your nose and eat a jelly sweet. You get sweetness but no flavour until you let go.