Opening lids?

Lili • Twin mama ~22~ Married 💍

So I have 16 month old twins and they have recently figured out how to open their sippy cup lids. They just twist it off and dump it everywhere. Especially their bed. They have a water sippy cup in their bed for naps/bedtime, and tonight one twin twisted the lid off and soaked his bed and clothing with water. How do I stop this? 😭