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Yea. And fuel can be many things. In Denmark we say rye bread motor. Used when people use their body to move around - bicycling, walking, skiing... one of those strange word combination that you seldom think about. Except I just did today when I was out walking and saw one of the rent-bikes who had it written on it as a promotion-slogan.

Love that: rye bread motor. It's quite perfect for human power in Denmark.

It was probably a joke and a smart and funny term in the 1920s :) Now it is just an idiom that you (almost) not notice. I often find it fascinating when peculiar things like that freezes in its form. I of course mostly notice in other languages where I am not as used to the imagery used.