Wonder if water balloons would be considered hand grenades?
At the school where I used to teach, there was a tradition that on the 8th graders’ last day, (it’s a K-8) district, there would be big water ballon fights on the buses on the way home. For safety reasons (and killjoy reasons) we teachers would try to ferret out their hidden ammunition depots. In spite of our best efforts a few would make it through. So, we then switched to not telling the 8th-graders when their last day would be. After a few years of that, the tradition disappeared.