Zinkrohedd was right.  Here's a few things to think about when putting your 
SolarShower (tm) or SunShower (tm) in the bin up top.  

1.  Put a layer of insulation under the bag.

2.  If you're traveling the convective cooling of the air will essentially
    prevent any heating up so put a transparent cover over the bag and make
    sure there's a still air space between the cover and the bag.

3.  If parked keep the bag out of the wind.  Use your favorite method.

4.  Test the water before showering.  It can be very hot if you've insulated
    the bag and kept it from convective air cooling during the heating 
    process.

5.  Do not heat the bag by placing over a magnesium aluminum alloy fire.
    The water will cause the fire to burn much brighter than you expected.

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         ___  / __   __/   ____/         Al Knoll HP Performance Technology /
        /    /      /     /             Center    Roseville, CA, USA 95747 /
       _____/      /     /             916.785.9453    (Telnet 785-9453)  /
      /           /     /             email:  alf@hpptc38.rose.hp.com    /
   __/         __/   ______/         ___________________________________/