Ask him "why not?" - it's a variation on asking them what they think, just so you don't get bored with the same response Works in this house!

It can be irritating. I think sometimes it's about keeping the parents attention, controlling the flow of conversation and learning how to HAVE a back and forth conversation. Remember that in his eyes, you know the answers to everything lol so if you're unsure or giving vague answers (cause there isn't really an answer) he might be pushing cause he thinks you really do know.

The other alternative, along the lines of what Alibaby posted, is to give over the top, exaggerated silly answers when his why's get silly. DP does this a lot and it usually ends in laughter and a change in subject or focus.