In basic terms, the moon pulls fluids because they’re “easier” to pull.
In slightly more technical terms, the apparent influence of the moon’s gravity on oceans is because 1) oceans are literally massive, gravitational pull is based on mass and 2) oceans are fluid, so it’s easier to make them move around. It’s worth noting that this also happens with our atmosphere, just to a lesser extent cause our atmosphere isn’t quite as massive as our oceans. The moon still pulls on solids, it’s just less noticeable cause they aren’t fluid.
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