tide
most people grow up thinking that the tides are caused by the moon and indeed that gravitational pull of the moon is a major factor as is the gravitational effect of the sun but there is another major factor which is less often mentioned an that is the force created by the rotation of the earth itself.
as the earth roatates on its axis once every 24 hours if the moon stood still then the moon will be overhead and underfoot once in every 24 hours giving (in most places) 2 high and 2 low tides a day but as the moon is also orbiting in the same direction as the earth revolves it actually takes about 24 hours and 50 minutes for the moon to be in the same place overhead.
so the period from one high tide to the next is about 12 hours 25 minutes.