Among other things, Darren New saw fit to write:

>> That's it. The solid earth can be considered rigid, while the water in
>> oceans obviously isn't.
> 
> Except you get tides even when a planet is entirely made of liquid. So
> that's not really it.  It took people a long time to explain why there
> are two tides per day instead of one (given that such was noticed
> thousands of years before Newton was alive), and many of the old
> incorrect explanations still float around as "lies to children".

Yes, but we "notice" tides in relation to the solid earth, so the liquid
sphere being deformed while the solid one remains rigid is what we call
tide, isn't it (in a broad sense)?

Anyway, I agree my explanation may not be the best one, and the differential
rotation speed could be a "more true" one. I recall this was beautifully
explained by Isaac Asimov (back to science fiction) in one of his writings.
Also, it is the tidal force (wich in truth also works on solid bodies),
which has made the rotation periods of the Moon around itself and around
the Earth to match, so that we see always the same face of the Moon. Which
could be a bit harder to explain is the marvelous chance of the Moon not
only having almost the same apparent size as the Sun from Earth, but being
sometimes a bit larger and sometimes a bit smaller, so that we can see
total as well as annular eclipses. We are so lucky!

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