there is no third dimension, either. Now, by that I don't mean that there aren't three

dimensions… there are, but just we can't tell these dimensions apart: there's no special

direction you can call "the 1st dimension," [north? east? up?] so it doesn't make sense

to say "this is the 2nd one," or "what's the fourth one?"

It's like if you have a jug with three cups of water in it... it's easy to tell that there

ARE three cups of water, but which is the first cup? the second? or the third? Those

questions don't even make sense!

That said, there may be different KINDS of dimensions that you CAN tell apart - like

if you poured three cups of water and one cup of oil into our dimensions jug. Then you

could say "I have three dimensions that behave in the same way, and one that behaves a different

way." And that's what we mean when we say that we live in three spatial dimensions and

one time dimension.

And there's nothing crazy about adding another cup of water so that you'd have four watery

cups and one oily cup; likewise we can contemplate what the universe would be like if there were

FOUR spatial dimensions and one time dimension… or in the case of string theory, nine spatial

dimensions and one time dimension.

And while we're at it, it's not even too hard to conceive that there might be two dimensions

of time… but I don't have even have enough of one time dimension to explain that now.

So if you'd like to try your brain at visualizing a world with four spatial dimensions, check

out the book Flatland or try solving a 4D rubiks cube.