when the piston of a fountain pen with a nib is dipped into ink and and the air is released by pressing it, the ink fills in the pen. why . give reason

Please find the answer in the explanation

Explanation: According to the Newton 3rd law of motion which state that;

In every action, there will be equal and opposite reaction.

when the piston of a fountain pen with a nib is dipped into ink and and the air is released by pressing it, a force is applied which act on the molecules of the ink by pushing the molecules. When the force is released, there will be a reaction force by pulling the molecules of the ink into the pen. Thereby the ink fills in the pen. 

asthenosphere and convection

a is the best answer your welcome