Shampoo cleans, conditioner protects. I sound like a commercial.
Shampoo is soap for your head. Soap takes greasy things and makes them dissolve in water. It does that by using molecules with a charged end and a greasy end. The greasy ends gather round the grease molecule in your hair, and the charged ends like water. The effect is to carry away the grease down the plughole. Awesome. Clean. Fresh. Ready for the day.
Some shampoos take away a bit too much. They can remove things that are good for you, actually (we didn’t EVOLVE with shampoo, so it must be possible to live without it without dying from scratching your head too much). The chemicals in the shampoo, none of which are harmful to you, might take away stuff that’s good for your hair. Some natural oils, some other things.
What you can do is lay down a layer of molecular stuff on the hair that covers over those things that have been damaged. I’m being vague for two reasons:
1) there are lots of different things you can put in conditioners – depending on what you want to achieve. Some of the molecules will paper over little molecular problems by acting like glue between breaks in the hair. Some molecules are shiny when they stick to the hair. Some reduce charge, or alter pH. Depends.
2) In some cases the reasons for the effects aren’t known, and people are using ingredients in conditioners because they’re known to do something nice, without worrying about the detail.
However, the key is that conditioner adds something in. You add a bunch of stuff to your hair and a lot of that stays on the hair, even if you rinse. So shampoo takes away, conditioner adds in. Sounding like a commercial again.
The conditioner bottle is upside-down. The lid goes at the bottom while it is on top on the shampoo bottle.
Shampoo was invented to clean the hair and prevent it from smelling, itching, and growing things.
Conditioner was invented a couple of hundred years later as a way to make more money. Someone told us that our hair needed to be shiny and easy to comb, and that conditioner would make all our dreams come true. Sure does!