[cmccart]

Ridiculing genuine curiosity is a terrible behavior from teachers... why stifle inquisitive minds? Anyway, you may already be familiar with these proposed biochemistries, but there has been a lot of speculation on this exact question over the years:

https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi...

[booleandilemma]

I think it's simply that the teacher feels insecure.