An even number is a number that you can divide by 2, and the answer will be a whole number (no fraction)
16 is an even number, because if you divide it by 2, you get exactly 8.
17 is an odd number, because if you divide it by 2, you get "8 and a half" (or "8, with a remainder of 1")
either way, 17 is NOT divisible by 2, therefore it is "odd".
In natural numbers (counting numbers) the numbers alternate: even, odd, even, odd, even, odd,... forever.
Therefore, an even number is ALWAYS followed by an odd number, then that odd number is ALWAYS followed by an even number (and so on, until you get sick of it).