I had a stout that refused to become fully carbonated in the bottle after 3 1/2 weeks. I opened a couple and they were good but not great because they were still a little flat. I read all about this problem on this site's forums and finally figured I would give something a try. I shook up the bottles and then placed them beside a warm (not hot) radiator in my spare room. Voila! Nicely carbonated beer. I wouldn't suggest this if you already have a nicely carbonated beer that you would like an extra pinch of foam out of. You may simply create bottle bombs. However, if you like me, had a stubborn batch give this a try. It seems as though the shaking suspends the yeast and the heat kicks them into action. Together I noticed a vast improvement in 3 days! I'm now leaving my next batch in the same spot and giving them a test squeeze (PET bottles) to make sure they're not going to pop.