As a regular coffee drinker (some may call it a caffeine addiction, but I'm not a fan of that term), Smart Lid seems like an ingenious idea to me. Having come across this piece years earlier would have saved me a lot of burnt tongue incidents.
How Smart Lid works is simple: the lid comes in brown, and when you slip it onto your coffee cup (it fits onto most regular plastic cups), it changes to bright red, meaning the coffee or tea is too hot to drink. When it cools down a bit, the lid turns brown again, letting you know you're good to go. Smart, right? Hence the name.
Smart Lid also comes in a green lid that turns white when your beverage is too hot. What's even cooler is if your lid is on properly, a green ring forms around the lid.
I need to get my hands on this immediately.