"As you know, this little grain of sand has mass. A very small mass, but mass nonetheless." And because this grain of sand has mass, it therefore exerts gravity. Again, too small to feel, but there." Now," Katherine said, "if we take trilions of these sand grains and let them attract one another to form... say, the moon, then their combined gravtiy is enough to move entire ocreans and drag the tides back and forth across our planet."