Yes, Terry gave a good description. It's just because when we speak, we tend to automatically make the contraction and is, was, has, does all end in 's.'
He's done that before. = He has done that before.
Where's that go? = Where does that go?
There's nothing in that box. = There is nothing in that box. (The contraction here could be ambiguous, is or was, but we'll be less likely to mean 'was' when it's in the contracted form.
When's the first time you drove a car? = When was the first time you drove a car?
The 'is' and 'was' variants can be ambiguous, but look at the rest of the sentence to determine the tense.