Beep-beep-beep-beep: You're in a hospital, construction zone, school campus, city street. Pretty much anywhere, you're surrounding by a ubiquitous high-pitched chirp. It's supposed to alert you to something, but it's so ever-present that you've learned to tune it out, just to keep your sanity.
Furthermore, is it just me, but I have a hard time locating high-pitched sine waves especially if they are really short...?
So, here's my invention. It's simple really: A no-beep microwave!
Yes, that's it. Million dollar invention, right there.
1. You'll know when the microwave stops humming.
2. You'll smell it.
3. You put it in there 3 minutes ago. How short is your attention span?
Or how about this idea (at the risk of making microwaves even more complicated than they already are): Have a setting for how loud you want it to beep, or if you want it to beep at all. It could even be turned up really loud, like a foghorn, so you can let the whole apartment know that you're having nachos.