Beware they may leak

Ive purchased and own many of these sippy cups. They are great for milk when you are on the go but want to keep it cool. They have a hard spout. Now i do like them and will buy new ones for my newest little person and as gifts. My issue is that some leaked straight from the store. The plastic valve that is in the spout just wasnt right and if the cup was on its side it would drip. Amd with little people who dont yet to always keep the cup up right or is a lunch bag or cooler where it will most likely end up on it sides it could make a big mess. But i return them and got different of the same cups and they were fine. So check them for leaks before use. Also they will leak if you turn them completely up side down and shake but most sipply cups spill proof or not will leak if you shake them