Here's why:

* DNA is normally loose and stringy: In the nucleus of a cell that's not dividing, DNA exists as long, thin strands called chromatin.

* DNA condenses for division: When a cell prepares to divide, the DNA replicates and condenses into compact, X-shaped structures called chromosomes. This makes it easier to separate the genetic material evenly between the two daughter cells.

So, while the actual DNA molecule is the same, its structure changes during cell division and we call it a chromosome.