Snowmobiling in Yellowstone at the end of the season.  We had the trails to ourselves – which is actually a little risky., as there was no one to winch us out.  Our only problem was that the engine overheated, so we had to keep stopping to pack snow around the engine housing to cool it.  We ended up snowmobiling after dark for the last couple of hours, which was EXTREMELY cold.  My teeth didn’t stop chattering for an hour!