Well... In plain English I would explain it in such a way:

Imagine 2 people, Alice and Bob. Alice knows only how to add and subtract numbers, but Bob knows how to add, subtract and multiply them.
If u ask Alice to calculate 2*3, she will do it in such a way: 2 + 2 + 2. Or 3 + 3 if she is smart enough. Obviously, Bob would just multiply these numbers.
Now let's ask them to calculate 2.5 * 3.72. Alice won't be able to do it, but Bob will do the same without any problems.
U see there is a difference between Alice and Bob, coz Alice, unlike Bob, is Turing-incomplete.