You can say that someone is still wet behind the ears, implying that they haven't finished drying out after birth. This is used to say that they do not understand yet, or are not ready for a task.
According to Wiktionary (but I have not verified the sources), its etymology is from
c. 1850, Pennsylvania, calque from German feucht hinter den Ohren.
From the drying of amniotic fluid on a baby after birth, specifically a new-born farm animal, which last dries behind the ears (partly because licked dry by mother everywhere else). German variants (still wet behind the ears, not yet dry behind the ears, green behind the ears) also borrowed.
According to this clipping quoted in "Green behind the ears": the untold story, by Ben Zimmer at the Language Log, the expression has been an Americanism since at latest the 1870s:
The Newcomerstown "Eye," a new paper, has already got into a squabble with an editor named Persinger, at Bloomington, Illinois. The Bloomington man should wait until the Eye gets dry behind the Ears.
— (New Philadelphia) Ohio Democrat, May 2, 1878, p. 4, col. 3
You have to love the tongue-in-cheekness: "The Eye gets dry behind the ears".