The Conway Knot's Slice

Mathematicians think of a knot as existing in a piece of string with two ends. But our world is four-dimensional, if we include time as a dimension. Three dimensions provide enough room to build knotted loops, but not enough room for them to unravel. Four dimensions provide a similar environment for knotted spheres - which mathematicians first constructed in the 1920s. Any knot you can make by slicing a knotted sphere is said to be a slice. Some knots are not slices: For instance, the three crossing knot known as the tree foil. It's hard to visualize aknotted sphere in 4D space, but it helps to first think about an ordinary sphere in 3D