Feet smell and feet sometimes smell bad. Feet smell because animals, including humans, produce chemicals that identifyus as individuals.
Feet smell bad when very tiny plants or animals grow on our skin.
It sounds gross, but it's true. There are plants and animals so small we need a microscope to see them. There are billions and billions of them and they live everywhere - even on us. They even live inside us. The microbes that make feet smell bad have very long names.