The Mash Tun is a 10 gallon Rubbermade cooler.  The cooler has a removable grain screen and brass fittings for  your mashing enjoyment.  Mashing in a cooler is superior to a stainless steel keg for two reasons.  First, it doesn't weigh as much, and second it is insulated.  The insulation allows you to reach your target temperature more quickly, and use less energy to hold it there.