What causes the tides?

The moon's gravitational pull is stronger on the side of Earth facing it, drawing ocean water into a bulge. The puzzling part is the second bulge on the far side, which exists because gravity weakens with distance: the moon pulls Earth's solid body more strongly than it pulls the water on the far side, effectively leaving that water behind. Both bulges stay roughly aligned with the moon while Earth rotates beneath them, which is why most coastlines pass through two high tides and two low tides in a little over 24 hours. The cycle runs about 24 hours and 50 minutes rather than exactly 24 because the moon has moved along its orbit in the meantime.

The sun contributes too, at roughly 45 percent of the moon's tidal effect despite being vastly more massive, because tidal force depends on the difference in gravity across Earth rather than on gravity's absolute strength, and that difference falls off sharply with distance. When sun, Earth, and moon align at new and full moon, the effects add and produce the larger spring tides, a name referring to the water springing up rather than the season. When the sun and moon pull at right angles at the quarter moons, they partly cancel and produce the smaller neap tides.

Local geography matters as much as astronomy for what you actually observe. Ocean basin shape, coastline configuration, and water depth can amplify or damp the range enormously, which is why the Bay of Fundy in Canada sees tidal ranges exceeding fifty feet while parts of the Mediterranean barely register a foot. Some locations have only one high tide per day, called a diurnal tide, because of how the basin resonates. Tidal friction is also slowly transferring Earth's rotational energy to the moon's orbit, lengthening our day by a couple of milliseconds per century and pushing the moon about 3.8 centimeters further away each year, measured precisely using reflectors left by the Apollo missions.