Every restaurant pays somebody to haul away its used fryer oil. Run right, you are the one getting paid, twice: once by the kitchen, and again by the refinery that turns it into fuel.

A leash and a phone is the whole startup cost, which is exactly why everyone assumes dog walking has no ceiling. One operator sold hers for seven figures. The difference was never the walking. It was the routing.

A guy with zero industry experience bought an 80-year-old septic company doing $1M a year and tripled it in two years. He didn't invent anything. He added routing software to a business that had never used any.