Tides in the sea are caused by-?

Tides result mostly from the mutual gravitational attraction between the earth and the moon. The sun's gravity plays a part in magnifying the effects of the tides, but over-all the moon's effect is stronger. The part of earth facing the moon, and the opposite part of earth (180 degrees of longitude away) have similar tides. This is counter-intuitive, isn't it? How could there be high tides on opposites sides of the planet? The earth and moon can be roughly thought of as a double planet, or binary system. The earth and moon orbit around each other in a sense. They actually orbit around their mutual center of gravity, the barycenter. Because the earth's mass is so much greater than the moon's, the earth-moon barycenter happens to be within the body of the earth. Have your big, jolly and friendly uncle go out in the yard with you with a length of rope. Each hold an end, and carefully start swinging around each other as you pull on the rope. Chances are, you will be moving a lot; Uncle Joe may not have to move much, but the faster you go, the more he will have to make a circle with his center of gravity in order to keep you in your circle. As the earth and moon orbit the barycenter, the same thing happens; you would witness a wobble in earth's movement that is keeping step with the moon's orbit. The 'back' side of the earth pushes out, adding to the effect of tides on the side farthest from the moon. Also, since that part of earth is farther from the moon, the moon's over-all gravity is less strong than it is on the moon-side of earth.