Rabbi Menachem Mendel of Lubavitch, the Tzemach Tzedek, related on the eve of Rosh Hashanah: Once I met some Jewish soldiers, who told me that they had the custom to say Psalms while polishing the buttons of their uniforms for inspection. The buttons are polished with sand and water. Sand is the letters of the Psalms. Water is cleansing tears. Everyone must purify himself by means of saying Psalms with tears from the depths of the heart, and simultaneously, with the joy of the soul.

(Sefer Hasichot 5705, p. 3)