Pebble, a smooth, rounded stone usually less than 2 inches (64 mm) in diameter. (A larger stone of this type is called a cobble, or cobblestone.) Pebbles are fragments of any kind of rock that have become worn by rubbing together in rivers or on beaches. A loose accumulation of pebbles is called gravel. Conglomerate is rock composed of pebbles cemented together.
He was born exactly 300 years after Galileo died. He has yet to be awarded a Nobel Prize, although he was awarded a guest spot on “The Simpsons.” What else do you know (or not know) about this acclaimed physicist?
So much about galaxies remains a mystery. We know what they're made of and that we live in one (the Milky Way), but we're not sure how they form and evolve.