I have had my favorite mugs over the years; some just seem to fit my hand and lips better. I used to heat water in a mug in the microwave, but notice that over time, (years) the mug gets way to hot to handle. I wonder why this is? Same microwave, same mug, same time, same amount of water. The longer it is used, the hotter it will get when microwaved. Any scientists here who can help?