The soft sand that we sink into on the beach, is actually rock. Sand is what a rock becomes after years of being worn down by rivers. Years of sea waves crashing against huge rocks and cliffs makes rocks break into small particles. And ultimately, they end up as sand. The colours of sand -- yellow, red, grey, black -- depend on the kind of rock it comes from. Sometimes, desert sand is carried by winds across great distances, to seasides, increasing the amount of sand in the sea.