Kids and Sand

Why do kids love playing with sand so much? Could it be the feeling of lush between their fingers? Or the ideas they play in their head of the things they will build? I love sand as much as these kids, but today was a mobile type of activity kind of day. I was picturing running around and going down the slide with them. It's sunny, but cold... and my youngest is once again having tummy issues. Of course, he isn't running around. He is playing with his cars on the sand, totally being immobile. I guess at least he's happy.