We have introduced tides in our earlier discussion of the Moon's observational characteristics through the effect of the Moon on the Earth's oceans, but the effect is much more general, and has a number of important consequences.
There is a limiting radius for the orbit of one body around another, inside of which the tidal forces are so large that no large solid objects can exist that are held together only by gravitational forces. This radius is called the Roche Limit. Thus, solid objects put into orbit inside the Roche limit will be torn apart by tidal forces, and conversely, solid objects cannot grow by accreting into larger objects if they orbit inside the Roche limit. A famous example is the rings of Saturn: because they lie inside the Roche limit for Saturn, they cannot be solid objects held together by gravitation and must be composed of many small particles.
Obviously solid objects can exist inside the Roche limit (for example, spacecraft) but they must be held together by forces other than gravity. This is true of a spacecraft, where chemical forces between the atoms and molecules are much larger than the gravitational forces.
Thus, billions of years from now the Earth will always keep the same face turned toward the Moon, just as the Moon already always keeps the same face turned toward the Earth. We will encounter other examples of such tidal locking in other pairs of objects in the Solar System.