As non-chalantly as possible, pull out a stopwatch and say, "You have to do these wallsits."
Kid: "I don't want to do them, I'm not going to."
Parent: gazing at stopwatch "You got 10 seconds to hop on over there against the wall bud. Otherwise it's white rice for dinner for a week."
Parent: "Grumbling and pouting is a bad habit, as well as laziness. Gotta get it out of you."
This should be a standard for discipline, but parents let this and everything else slide, because they don't know how to raise character.
Probably because they don't have much of it to pass on to begin with.