ideas factory: #42 : The time-limited tea bag

Can't someone make a teabag that stops brewing after a few minutes? After the allotted time, the paper in the bag shrivels up and stops the tea brewing any further. Or the bag fills itself with air and floats to the top. Or the tea is specially treated so that it dissolves as it brews, so after a few minutes there's no leaves even left.

Why the hell has no-one done this yet? How can people spend so much time on terrabytes and particle accelerators and whatnot, when there's so much of the basics to sort out?