This topic's one that I use when I'm talking with year 10 students about critical thinking: ear candling. It involves the close approach of ears & candles. Hollow, burning candles.
No, seriously. Ear candling involves inserting a hollow candle (made of cloth soaked in paraffin or beeswax) into someone's ear, while they're lying down, & then lighting the candle & allowing it to burn down until it's about 5cm from the ear. Supposedly the burning candle generates enough suction (hot air rises, after all) to draw wax & other sticky bits & pieces out of the ear canal, with all sorts of claimed health benefits. And how do you know that this does indeed clear gunge from the ear canal? Well, when you cut the candle stub open, it usually has all this brownish waxy residue inside...
When I first came across this I thought of two words to describe why this is probably not a good idea: 'hot' and 'wax'. When candles burn they produce a lot of molten wax and, if you've ever had this drip onto your skin, you'll know that it hurts! Molten wax dripping down the ear canal & onto the eardrum - it sets my teeth on edge just thinking about it.
Anyway, the students & I talk about this & then I ask them to come up with an experiment that would allow them to determine whether the residue seen inside the candle stumps did indeed come from within the ear. Someone always comes up with the method that's been used to test this claim scientifically: candling a volunteer's ears, & burning candles in the same way but outside the ear. (Some researchers have used an additional test, candling the volunteer's ear but using a one-way tube, so that wax could come up from the ear but not down from the candle.)
And what did the researchers find after their tests? The stubs of candles burned away from an ear contained exactly the same sticky waxy residues as candles from volunteers' ears - the gunge came from the candle and not from the ear canal. (In any case, wax is very sticky - you'd need a very strong suction indeed to drag it out of the ear canal & this would probably rupture the eardrum in the process... Yet burning candles produce no such suction.) So this technique will generate a certain amount of heat - but not a lot of results.