Entertainingly, this myth was 'disproved' by the BBC in 1991, using a device that flipped a slice of toast in the air, rather like flipping a coin. Although it wasn't quite 50:50, not entirely surprisingly, on the whole the buttered and non-buttered sides ended up downwards roughly evenly.
But what the producers seemed to have missed is that there isn't much toast flipping going on in our kitchens. What usually happens is either that toast slips off a plate in our hands, or off a worktop. Both of these tend to occur at around waist height. And without a forced spin to get them going, the chances are high that the toast will only have time to revolve half a turn in the fall. It usually starts butter side up (that's certainly how my toast goes on the plate)... so it ends up butter side down.
To accompany my talk based on the book I made a very short video demonstration of this. It seemed a shame to limit this to those who had a chance to come along and see the talk - so, for your delectation, here it is...
Image from Unsplash by Seriously Low Carb
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I mentioned a similar explanation in my Fake Physics book, referencing an Ig-Nobel prizewinning paper from 1995 (R.A.J. Matthews: "Tumbling toast, Murphy’s law and the fundamental constants", Eur. J. Phys, 16(4), 172): "According to Matthews, the actual presence of butter on the toast is irrelevant to the dynamics of the problem, except insofar as it defines the “top” side of the toast in its initial configuration. He then argues that, as the toast falls a typical distance to the floor (e.g. from a table-top), the laws of physics imply that it will complete less than one revolution and hence land the other way up."
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