The cap works to keep liquid sealed inside the bottle. We have traveled with water already in the bottle so we can just add formula on the go and we haven't had any leaks. The cap secures with a click over the metal ring. So you need to take the cap off and then unscrew the ring/nipple to open the bottle. Also the cap is not flat on top, it is almost, so it wobbles if you place it upside down. I mention this because it would be nice if the cap top was flat on top so I could place the ring/ nipple inside while I add formula to the bottle while I'm on the go, without the risk of it falling over.

Love these bottles and top

Love that this product is even offered! Really makes the mason bottle complete!

I love how secure they are on the bottle so I never have to worry about them just falling off and the nipple getting dirty

It is awesome that these caps allow me to use the mason jars I already have instead of buying new. My only complaint as a busy mom would be that they are tricky to get open one handed, but at the same time I am so glad they protect the milk inside from spilling out that it’s a problem worth having.