Positive Commandment 13 (For Children)

Deuteronomy 6:8 "And you shall bind them for a sign upon your arm"

Besides the tefillin on the head (see above, Positive Mitzvah 12), the Torah commands men to put tefillin on their arms.

The square leather box is placed upon the arm, facing the heart.

This shows that our feelings should be connected to HaShem, just as the tefillin are tied onto us.