To Rub
To Scrub
transitive verb, godan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means rub and this verb means to rub or to scrub.
If you're going to rub me like that, I'm going to sue (す) you. It's making me very uncomfortable. The cost (こす) of your rubbing actions will be high.
So this word has two readings and both mean “to rub.” But you usually use する for a single action, such as lighting a match or grazing your knee, while こする means doing it repeatedly, like when you’re cleaning or polishing something.
花粉症で目を擦ってたら、片方のコンタクトレンズが破けました。
I was rubbing my eyes because of my allergies and one of my contact lenses was torn.