The first letter T is mapped to Y, that is 5 places later in the alphabet. The second letter H is mapped to N, 6 places later in the alphabet, and the third E to L, 7 places. So that it seems that each time we code a letter, it is mapped one more place later. Now D is mapped to M, that is 9 places later. Obviously the space caused an increment. This way we predict that the A in ATE would be mapped 13 places later or into N and that is indeed the case. So we have broken the code! (Or rather almost, because there are a few more "details" to take care of.)