Four. O, double-u and e.
It must need Osteo-per-o-sis" Love it.
Student: H, I, J, K, L, M, N, O. Teacher: That's not what I taught you. Student: But you said the formula for water was...H to O.
Punishment.
Justified. (NB Sincerely hoping that this is actually original)
Aye E, I owe you...