ELI5: Why do pens dry out when the cap is left off, but the caps themselves have holes?

The pens dry out because the ink contains solvents. The solvents are volatile and will evaporate if exposed to the air too much.

The caps slow this down but they don’t completely prevent it. The solvents will still evaporate but will largely just sit inside the lid if there’s no air moving past the pen.

The holes in the lid are there so children are less likely to choke and suffocate if they swallow one.

The pen cap holes are also useful to pass a balloon catheter through the cap to aid the removal of the cap from the child.

Explain Like I`m Five: good questions, best answers.