Asked by: H Hughes, Pembrokeshire
Every water molecule feels exactly the same gravitational pull from the Moon, regardless of how many other water molecules surround it. But that individual pull is very small and is completely overwhelmed by other short-range forces, including electrostatic interactions between molecules. Gravity is an unusual force, though, because it has no positive and negative versions; it only attracts. This means that the net effect gets stronger the more molecules are in one place, rather than just averaging out to zero.
Lake Superior in North America is the third-largest freshwater lake in the world, by volume. But the 11,600km3 of water it contains are only enough to give it a tide of around 2cm. Tides any smaller than this are very hard to measure because they’re masked by the effects of weather and river flow levels.
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