I have a Far Side mug I got as a gift over 20 years ago. I was nuking some lemonade in it, and the glaze on the handle and the rim, got all cracked, or crackled. So I put it aside thinking that was the end of the mug. Today I see it back in our cabinets and you can't tell anything had happened. It's like the glaze uncracked.

How did this happen? And is the mug still safe to use? I figured I wouldn't nuke anything in it anymore, but what about drinking hot liquids? Or any liquids?