If you make coffee using an Aeropress (recommended)
Use the inverted method (recommended by many, though I've never really had it work)
Spill coffee grounds on the bottom of the tube because you're not using the funnel.
It is, theoretically, possible to clear the grounds off by blowing on them. In practice, however, you're likely to accidentally blow into the tube of finely ground coffee. Which will end up in your eyes, the coffee mug, the floor, etc.