I agree with Catina above. My son did this ALL the time. He is also 5. The years between 2-4 was unbelievable with the amount of stuff he would throw. He still does it now, but not as bad. He started with rocks, then sticks, leaves, toys, anything he got his hands on. We basically had to prompt him "no throwing". If he threw a toy, he did not get it back (if you pluck them out). If he continued to throw, he would have to sit in his "think about it chair" as we had the conversation about "no throwing". I found in the beginning, it was very sensory driven. The visual stimulation of water was so high for my son and sometimes still is to this day. We avoid going to certain parks near streams, lakes etc. (he calls them rivers) to avoid it since he has a hard time leaving those places once he starts his throwing. It is also very OCD. They remember doing it and come to a compulsive conclusion that, that is where I want that. And it is simply needing to show that those items do not belong there. We had to go as far as putting up a temporary fence (we live on the lake) to prevent him from having direct access to the water.