Here at Nalgene we have very high standards when it comes to calling a container leakproof. We fill up the container and then pressurize it. If it does not leak we consider it to pass the test and be worthy of being called leakproof. We test the jars by filling them with water and then turning them upside down. If no water comes out we consider the jar to pass the test. So jars will hold liquid but they are not held to as high a standard as the bottles. The jars may be a better choice for thicker liquids such as lotions or gels. And of course they are great for Tums, or Advil.