We bought a book.
The first reason that rocks are round is erosion. If you bang a spiky thing against another spiky thing, the spikes break off first. If you rub a rough thing against another rough thing, they both get smoother. This is the only reason I ever considered, round-rock-wise.
The second reason rocks are round is much, much more interesting. If you have a big huge piece of molten rock and you let it cool, it will cool unevenly. The molten rock is more like chocolate chip cookie dough than whipped cream, and some parts will solidify before other parts. Sooner than you’d think, you’ll have suddenly solid parts surrounding still liquid parts. The still liquid parts will get rounder and rounder as they cool and harden from the outside in. In addition, whatever cools first will be more messy and mixed-up, and will probably be weaker than the last parts to cool.
So, when the huge piece of rock finishes cooling, it will begin to break where it cooled first, and break last where it cooled last, leaving round rocks surrounded by egg-shell rocks that quickly break into flat rocks that quickly break into sand and small rocks. Once you see it, you see it everywhere.