The next mitzvah performed by Jewish men after donning tzitzit is putting on tefillin. (May women? That would take more I have room for in this column.)
Tefillin are black cube-shaped boxes that are bound with leather straps on the left arm (for the right-handed) facing the heart and on the forehead. Each of the boxes contains four biblical passages, including the first two paragraphs of the Shema, about the unity of God, written by a scribe in ink on parchment. These days people wear tefillin during shacharit, morning services; a few saints and sages wear them all day, but most of us cannot keep our thoughts at the appropriate pitch of holiness to justify doing so.
Tefillin remind us to devote our lives, heart and soul to God. Rambam writes: "The holiness of tefillin is very great. As long as the tefillin are on the head and on the arm of a man, he is modest and God-fearing and will not be attracted by foolishness or idle talk... but will devote all his thoughts to truth and righteousness."