I first learned about Swarfega more than 50 years ago. And I can't remember how I learned about it, which probably means that it came from my parents, which probably means my father, although since he died when I was five, I'm guessing that there was a tin of it at home.
When you work on a bike or a car, your hands become black with oil and grease. Soap helps only a little; to get your hands clean again, Swarfega is the way. I suppose there must be other products, but Swarfega is the one I know about.
You get a splodge of it out of the tin (now a plastic tub, or a pump-action bottle) and rub it all over your dry hands. Then you rinse, and magically, all the greasy dirt rinses off.
There's also an orange version, which A) has a nice orangey smell and B) contains a sort of gritty gritness (actually bits of corn grit, and therefore biodegradable) that helps scrub your hands.
I use both. You can get it cheaper on Ebay.
I recently dismantled the hub gears of my bike; the inside grease had become black and sticky, and my hands were really filthy with it. Swarfega made them sparkly clean again.