Ten dimensions.
It's not really something I think every day. But, I came across this video (linked from SfSignal, who got it from MySelfDev) and found it very interesting.
It's a 10 minute video - but you won't be bored. The narrative progresses from a single point to the sum of all possible universes and timelines in a very seamless, understandable manner. At the very least, you can now, with an educated air, correct your friends when they talk about the 25th dimension!
If you want to learn more after watching the video, there is a book available at "Imaging the Tenth Dimension".
Finally, here is my attempt to direct link the video. Hope this works!
[via myselfDev]