I don’t want to discourage my kids from asking why. However, I still want them to obey right away. So, the solution I have found to work is to have them obey first and then ask why. This is how is how a conversation might go:
Me: Abby, can you get your shoes on, please.
Abby: (Beginning to get her shoes on) Sure, why?
Me: We are going to the store right now.
When “Why?” is not asked first, I simply remind them to obey and then ask why. I will not answer their question until they have begun obeying.
