Why use "serious explaining" but not "serious explaination"?

Saying someone has some serious explaining to do sounds just a little intimidating or ominous.

A serious explaination just means straightforward and factual. You know, unlike some of the jokey answers you might see on a2k.

In the sentence "explaining" is the gerund of the verb "explain." It is an action that the subject will have to do.

"Explanation" is a noun that refers to the verb "explain." It does not function as a gerund.

". . . President Obama will have some serious explanation to do" is not grammatical.

They mean about the same.

To "have to make (or give) an explanation" has the same meaning as to "have some explaining to do".

If you say someone "has some explaining to do", then you are suggesting that the matter is serious.

If person A has some serious explaining to do then the speaker is saying, or it is being predicted by the speaker that others will say, that person A is required to explain why a failure or error, or poor performance occurred. To say it of someone is deliver an opinion or pass a value judgement on the action or event concerned.

Father (to son): I lent you my car and it was stolen! You have some serious explaining to do!

Son's friend: Jim borrowed has father's car and promised to take care of it, but it was stolen. When he sees his father he will have some serious explaining to do.

Thank you guys.
An apparently good usage.