What is the word for doing something absentmindedly?

Suppose a person throws his pen up and catches it by his one hand when he is deep in thought to solve a math problem. They don't know why they do it, but they just do and they do it unintentionally and inattentively. This behavior is generally not easy for a person to give up, but if one lessens his absentmindedness, he can stop this habit. What is the word for this behavior?

Solution 1:

I believe Instinctively or Instinctive is the word you are looking for.

http://www.thefreedictionary.com/instinctively

http://thesaurus.com/browse/instinctive

Of, relating to, or prompted by instinct.

Arising from impulse; spontaneous and unthinking: an instinctive mistrust of bureaucrats.

Solution 2:

I would say it's just a reflex.

an action that is performed without conscious thought as a response to a stimulus.

Or one could think of it as a subconscious impulse that forces you to act even before you realize it. That really looks like one did it unconsciously, but I feel in reality it's a subconscious act.

Solution 3:

I would call that behavior:

• an unconscious habit

• a mindless reflex

• a bad habit

• a compulsion

• a compulsive habit