What happens when you use a tea bag a second time?
I'm just posting this cause I'd always wondered what the answer was, but never cared enough to look it up. Thanks to /r/askscience, I now know the answer!
The numbers vary somewhat, but if you make 1 cup of tea with a tea bag you get most of the total extractable caffeine (around 70% or so). If you make a second cup of tea you get close to that last 1/3 that is left. Here is a summary table.