Why does my cat drop his food in the water?

I am perplexed...I feed my 5 year old dry food and always have. For a couple of years now he has been dropping a pebble or two in his water bowl. I saw him in action today for the first time and saw that once he drops it in his bowl he tries to lick up the food pelet (but never is successful). What do you think is going on? Thanks!