MEM, the Hebrew letter that equals the number forty, has an open and a closed form. The open MEM appears anywhere but at the end of a word. The closed final MEM is used to end a word.
The letter MEM begins and ends the word mayim (water), signifying the lifeforce.
In Jewish thought, the number forty represents the span of time necessary for things to come into fruition in nature. The forty years in which the Jewish people wandered in the desert.
The number forty also signifies the meaning of the mikveh (Jewish ritual cleansing bath), which must be forty seah (a unit of measure) deep.