Nalgene is well known for making long lasting and safe drinking bottles. These storage containers follow the same tradition - made of sturdy HDPE, PP and PETG. This travel kit contains a selection of 4 leakproof, durable bottles with independent screw-on caps and (2) storage jars. These are all Dishwasher safe. Nalgene has very high standards when it comes to calling a container leakproof. To test a container they fill it up with water and then pressurize it. If it does not leak, it is considered to be worthy of being called leakproof. 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, Advil or other medications.The jars are tested by filling them with water and then turning them upside down. If no water comes out it is considered to pass the test.