Thermal insulated. Lightweight. This stuff is as good as “sliced bread.” What? It’s blown glass – yes, there is a “blown” mark. The air insulation keeps the glass from transmitting heat or cold. Your coffee stays hot and your lemonade ice cubes don’t melt. Too good to be true. Colleen loved them and I got them – for her (how could I deny her…). I tried one. Oh boy! Light bulb! As in all things I overdid as you can readily see our cabinet with an assortment. Hey! That’s quite an assortment of tea sitting up there too. Good stuff.