Jim,
Sorry, its a habit.
And your not stupid for not knowing the Metric system, but our schools for not teaching and enforceing the Metric system, back to the point.
.394 inches per cm.
40 cm = 15.76 inches
13 cm = 5.1 in

Almost forgot that Jacks are not really sexually immature, and on the the other hand are very important to the spawning dance. They act as sattalite males, meaning thier size allows them to sneak in unnoticed by larger males and get thier genetics in with his and hers durring the act. This way if the male is infertile or somehow related to the female, you have two sets of sperm.

NEN cool
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Never Enough Nookie