Every 2 digit number is greater than 9 and less than 100

Let M and N be two two-digit numbers, then

 9 < M < 100
 9 < N < 100

We add those two inequalities term by term:

 9 <  M  < 100
 9 <  N  < 100
------------
18 < M+N < 200

So their sum is always greater than 18 and less than 200,
so their sum can never be a 4 digit number.  That's because
4-digit numbers are all greater than 999.

Edwin