Clean windows

We were driving around looking at house colors because we're planning to paint out house this summer.  There was a brief rainstorm outside and Karsten said 'Look Mommy, something is cleaning my window.' I was perplexed about what he was talking about, so it took a minute before I realized that he was talking about how a raindrop travels across his window and leaves a 'clean' spot.  That's a new perspective!