Actually, most of the time now, we use cheap water bottles, like what you get buying water at the gas station. When they are full, I suppose they would hurt making contact, but they don't stay full for long, my kids drink lots of water. I confess to using sippy cups when the dc were younger however. We have a van that has no way to secure things. I put groceries behind the 3rd row seat, but they could become projectiles too. There are cargo hooks back there, but they are not rated for holding things in a crash. There is no way I can eliminate all projectiles. I've toyed with the idea of putting a storage box in the back and hooking it to the cargo ties but I don't have any assurage they would hold in the event of a crash.