Made of durable and safe PP (polypropylene), these storage bins are strong and waterproof. The sturdy construction is designed to allow the storage box to retain its shape for long-term use. The bin has a collapsible design for easy space-saving storage for adults. It is combinable for use, stacking the boxes closely together. The top lid has four grooves that match the wheels, so you don't have to worry about the storage box stacking falling off. It can be folded flat and put away when not in use. Both the front and top lids can be opened for easy storage and removal of the contents. Moisture-proof, and dust-proof, with a two-way closure function. Four invisible rollers built into the bottom make it easy to move when doing cleaning. Great for closets, bookshelves, dorm rooms, closets, nurseries, playrooms, attics, laundry rooms, bedrooms, caravan storage, seasonal thin clothing underwear, or toy organization, etc.