Instruct: Classify Amazon reviews into positive or negative sentiment
Query: grows mold

Problem is the insert that prevents the cup from leaking grows mold very easily! Make sure you clean it regularly to prevent growth or keep it in fridge when it's too warm out. Empty every day before bed and wash immediately. It is best to prevent mold from growing on it - because cleaning the insert is close to impossible - in my experience. I'd recommend "the Safe Sippy" instead, or "Clean Kanteen".