* Injury: Gerbils are small and fragile creatures. A pen, especially if it's a ballpoint pen, can cause them injury. The pen could puncture their skin, scratch them, or even break their bones.
* Stress: The sudden contact of a pen could startle and stress the gerbil. This can lead to health problems in the long run.
* Unnecessary interaction: Gerbils are delicate animals and they prefer to be left alone. Putting a pen on them is an invasive and potentially harmful interaction.
Instead of putting a pen on your gerbil, try to interact with them in a gentle and safe way, such as offering them treats, providing them with a safe and stimulating environment, or watching them play.
Remember, animals deserve to be treated with respect and kindness.