The first "stone" that I will discuss is the humble "pebble". A pebble is the common word to describe a stone that is very small, like something that might catch in your sandal as you're walking on a dirt path. Though some would define it formally as a smooth stone, it is often used for any type of stone that is very small, even if it isn't smooth.
Perhaps the most versatile of these words is "rock". Sometimes a pebble is referred to as a "rock"; a boulder can be a "rock"; even the Earth itself gets called a "rock" sometimes, as in, "third rock from the sun". Usually, however, when we say rock, we are referring to something small enough to be picked up by one person but larger than a pebble. I know this isn't very specific, but that's just the way we use it-its a rather comprehensive noun and very useful since it's exact meaning is usually easily understood by the context in which we use it.
Again, while the definition may claim that a "boulder" is a large stone worn smooth, we generally use it solely to indicate the size of a single rock-a very large one. Although this next photo has a boulder in it, the term can also be applied to rocks much bigger than this one.
These next two terms are sometimes used to describe the same thing. While a "hill" is smaller than a "mountain", a person's perspective has a lot to do with which word he will use. Someone from the flat areas of the Midwestern United States, might call this a "mountain", especially since it has a craggy outcropping on one side, but after looking past this "hill" (its a hill by comparison) or turning around to face away from this place, one can easily see how small this hill really is.
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