The Moon exerts a gravitational force upon the Earth, but because this force is related to distance, it is slightly stronger on the side of the Earth facing the moon and slightly weaker on the away side. The solid body of Earth is only slightly affected by this difference. However, because the ocean is a fluid, the part of the ocean closest to the Moon is pulled more than the solid Earth, and the part of the ocean opposite the moon is pulled less. This is why there are two high tides and two low tides each day on the ocean.

Tidal forces are stronger at the time of the new moon, when the sun and moon are on the same side of Earth. The weakest tides come with the full moon. Lakes do not have noticeable tides because the difference in the gravitational pull from one side of a lake to the other is negligible. Likewise, a human body is far too small to be affected by lunar tides.

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