While standing in the HD plumbing section, I over heard the plumbing "expert" (A woman probably in her late forties who looked like June Cleaver)tell a customer that the difference between type L and type M copper pipe was that type L would freeze faster. When I tried to suppress a laugh she asked if she had that backwards. I explained to her and the customer the difference is in the wall thickness and inside diameter. As far as the freezing time you had better have a stop watch to tell the time difference.