The vacuum within the walls of flasks provides the main insulation to keep drinks either hot or cold.  This vacuum is either within a glass liner, a liner that needs the protection from an unbreakable plastic case, or within a double walled, polyethylene body like the children’s flasks on display.  The sealing lids and handles allow liquids to be carried to the picnic spot without being spilled, and the plastics materials used prevent temperature transfer from the outside to the contents, keeping it appetising, or from the contents to the casings making them safe to touch.

The examples here show how the design of flasks can be either practical or fun and provide a variety of dispensing actions from pouring into cups, sipping from a spout, or pumping from a standing flask.  Whilst the materials used provide strong colour and hygienic surfaces.