This otter is having a good time in the sun while effortlessly juggling some pebbles without even looking at them. This game is played by otters all over the world and has been documented numerous times. (another otter doing the same thing)
In the wild they are frequently spotted using rocks as crude tools to smash open shells, but it's not exactly clear what these guys are doing. They might learn to handle rocks through play, but several otter species that don't make use of stone tools also play this game. Otters are highly social animals and appear to engage in various behaviors, for sheer enjoyment and this seems to be such an occasion.