In four dimensions, you cannot tie knots in strings (you can move the loops in the fourth dimension so that it all reverts to the unknotted string). However, you can tie knots with two-dimensional surfaces.

And related to this (and since Houdini knew this fact)--Harry Houdini proposed that one could prove the existence of a fourth dimension by producing a pair of linked rings, with no evidence of cutting and rejoining, each ring made from a different type of wood. (he was wrong--it could be done with grafting, I think) since in four dimensions you can link and unlink rings without breaking them for the same reason there are no true knots of strings.

__________________
-----
Todd (Bowie, MD, US, North America, Earth, Sol System, Vega region, Local Bubble, Orion arm, Milky Way Galaxy, Local Group, Virgo A Cluster, Virgo supercluster, the universe in which spock is clean shaven)

Quidquid latine dictum sit, altum sonatur.

personal page: http://blog.astrosketches.info