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human
ELI5: how does lip balm work?

Chapstick, Carmex, whatever. “Medicated lip balm”. It is a moisturizer for lips, and lips only, it says. Other moisturizers say keep away from lips (and eyes, obviously, but lips are specifically named as well). So what’s the story?
gpt
Most lip balms contain a humectant, which is something that helps to draw moisture from the air and creates a barrier to lock moisture in. 

This is great for lips because you use them all day. Lip skin is delicate and if your lips become too dry or cracked it might make it hard to talk and eat, which are two things that are pretty important for our survival. 

Using a lip balm on normal skin is a bad idea because the barrier doesn’t allow your skin to breathe properly. If you applied lip balm all over your face then started sweating the sweat would get stuck under the balm, rather than evaporating.

When moisture and oils from your body mix under a barrier that stops oxygen from getting in, it creates an ideal environment for bacteria to grow. Too much bacteria and you can end up with pimples because your body produces white blood cells (pus) and inflammation (the redness of a pimple to try and prevent the bacteria from entering your body further. 

The strength of different types of moisturiser tends to relate to how often that part of the body is used. 

In order of use and strength:

Lips are used a lot, so it’s okay to use a strong moisturiser on them because it’s unlikely to stay there for a whole day.

We also use our hands a lot, wash them several times a day and we don’t normally cover them. Which means the skin loses moisture faster than the body can replace it. Hand creams tend to be quite thick and slimy so the protection lasts longer and has a better chance of getting into the tougher skin of your hands. 

Next, the skin on a face is almost always exposed. Our face moves and stretches a lot to allow us to make expressions, so a face moisturiser is like light but still fairly moisturising.

Lastly, skin on the rest of your body like your back, legs and arms. These parts of you don’t get bent, stretched, washed or exposed to the sun and air as much. They are also covered by clothes a lot of the time. So they only need a light and thin moisturiser at most because your body can usually manage the moisture levels on its own under clothes.