lines 1-2: Pens are wet on the inside because of the ink they contain. They are dry on the outside.
lines 3-4: Some pens have caps ("heads") that need to be removed before one can write with them.
lines 5-6: Other pens work by pressing their button, which often produces the sound of a "click". This "click" is not to be confused with the click of a computer mouse. Pens work via "human power" whereas computers need electric power in order to function.
lines 7-8: Pens are writing "instruments", not musical instruments. Without the aid of one's "hand", pens are "useless".