Many people don't wish to take full-size versions of toiletries, medications, or other items when they are traveling, so they purchase smaller travel bottles to carry these items instead. There are a number of different types of travel bottles available to store these different items in, featuring different types of caps for convenience. Though they are also available in many different sizes, it is important to check airline regulations before attempting to carry any bottles on a plane; they generally need to be under a certain size and stored in a clear plastic bag if they will be carried on as liquids, otherwise the airline staff at the gate will simply throw the items away.
Travel bottles exist for other purposes as well, such as holding beverages. These are typically also made of sturdy plastic or aluminum, and are often now free of bisphenol-A, or BPA, which is a concern in some plastic bottles. Travel versions of baby bottles exist as well. Larger bottles can typically only be taken on airplanes if they are empty, but they may certainly be used for other types of travel, and are better for the environment than constantly purchasing new bottles of water.