Here's why:

* Highly ionizing: Alpha particles are large and carry a double positive charge, making them very effective at ionizing atoms they encounter.

* Short range: Due to their size and charge, alpha particles lose energy quickly as they travel through matter. They can only travel a few centimeters in air and are easily stopped by a thin sheet of paper or even a layer of skin.

* Cannot penetrate sheet of paper: This is a direct consequence of their short range. Alpha particles are simply too large and heavy to pass through materials like paper.

Let me know if you'd like to know more about alpha radiation or other types of radiation!