Here's why:

* Nuclei are generally transparent: Cell nuclei are mostly made of DNA and protein, which don't absorb much light. This makes them hard to see under a microscope.

* Stains bind to specific cellular components: Stains are chemical dyes that bind to certain molecules within cells. Some stains, like methylene blue or iodine, have an affinity for DNA, which makes the nuclei appear darker and more distinct.

Let me know if you'd like to know more about specific stains and their uses!