"Yeah, it's a machine that uses soundwaves, set to a very specific frequency, that painlessly and gently dislodges plaque from teeth. It's so easy you can use it safely at home."
"Wow, that's amazing... But I think I'm going to pass."
"... Really? Oh. Can I ask why?"
"Yeah, sure. You see, we have this other tool. It's a metal hook. Cold, sterile, scratching, painful. It does nearly as much damage to the tooth and gum as it does the plaque. It's really invasive and fearsome and requires a professional hand not to just cut you to ribbons... And even then? You're going to bleed a bit."
"... I see. Uh, quick question: How is that better than my painless, soft, safe method?"
"It's not! But, well, I just really hate those damn darkies!"
"Urgh, yeah, say no more! Let me toss this shit in the bin. We won't even let our fellow, pure blood, white people use it! Anything to make a black person scared!"
"BWWWHAHAHAHA!"
"NNEEEYYAHAHAHAHA!"