Mathematically, 6 is the most interesting small number. For example, one might represent it as

  XXXXXX

  XXX
  XXX

  XX
  XX
  XX

  X
  XX
  XXX

or the sides of a cube, hexagons or six-spoked wheels. Occasionally someone would draw a rectangular prism with sides of length 1, 2 and 3. I often had students who might also draw 六 or 陸 or ٦ or ৬ (I had a diverse student body).

If any students hear the word wrong, you'll get some really funny drawings. (my guess)

Or if any of the students are from New Zealand

NZ = Sux

Aus = Sex

Most Kiwi accents I've heard noticeably pronounced other vowels as "i". e.g. "dick" instead of "deck", "tinnis" instead of "tennis"