The final group, the nitrogen-containing compounds, are unusual in that for some of them, their smell is actually different at different concentrations. Indole and the appropriately-named skatole are both found in human excrement, and contribute animalistic, faecal odours. However, at very low concentrations, both have pleasant, floral odours, and are even found in some flowers, and used in perfumes. Whilst we still don’t know about all the nuances of our sense of smell, it’s theorised that this is because it binds to different olfactory receptors in our noses at different concentrations – binding preferentially to one at low concentrations, and to another, through another part of the molecule, at higher concentrations.