Let me dust off my soap box. In my opinion "a journey down the rabbit hole" is exactly what kids need. It shows passion and interest in a topic. Something sorely lacking in today's education system. And, no, it doesn't mean that a child will "fall behind" academically in all other pursuits (common core is part of the problem here and not a solution). In the end every discipline of learning is inexorably entwined. As you approach each new, higher, level of learning about a topic you need to expand your ability to digest larger and more diverse sets of information. Which in turn requires deeper knowledge of language and mathematical disciplines. Snuffing out the inquisitive nature of children is, to me, one of the most heinous aspects of our education system.

I think the explanation is fairly simple. Grab two of the same item, I'm gonna use apples for this example.

First, hold one apple in each hand. Ask your child how many apples you have in one hand, them the other hand, then the total.

Then, hold two apples in one hand, with the other hand empty. Point out that you have two in one hand, and none, therefore zero, in the other. Have your child count the total number of apples, 2, and explain further how zero stands for no quantity. How when you have zero of something, you really have none.

Because zero is an additive identity. Might as well teach your kid number theory.

> rabbit hole

But why not go down a rabbit hole with a child, I feel it could be a wonderful experience that will challenge your beliefs and that of the kid. I don't really get the fascination with ignoring these type of questions: "Dad why is the sky blue" - "Eat your ice cream Timmy".

because child has to get homework done to go to bed or because sibling spilled some milk and there is no time to teach number theory to toddler who is trying to understand 1 + 0

"That's an excellent question. Ask me again tomorrow."

right - the operative phrase in the OP's post is Sometimes. "Sometimes you don't want to do that." He did not make a categorical statement, but y'all are treating like he did.

Because there are 15 other children in the classroom, and they may not get anything out of the discussion - and instead, may miss out on something important for later lessons.

You tell the child that that is a good, but complicated question, and it will take time to give them the full answer. Then you can introduce them to the pieces as they can understand them, and as you have time.

I don't see the problem. Tell them their question is a good one, but one that's very difficult to answer while providing a simplified answer (if you have a toy in one hand, and nothing in the other and you put them together, how many toys do you have together?). Critical thinking skills can be taught in way that's age appropriate. What's important, or so I believe, is that young children are taught to be curious and that it's OK to ask questions and be skeptical. They won't always get an answer, but that doesn't mean asking was wrong.