When toddlers ask “why”, they aren’t actually just looking for one single answer the way that adults are. Toddlers think of “why” as more like “tell me more about this topic”.
They have a natural, healthy curiosity about everything. And the worst thing you can do is to halt that curiosity, or make them feel bad about it in any way. The more curious we are, the more rich and fulfilling our lives are.
So it’s best to first answer the question as directly as you can, as you would for anyone else. And then, if there’s another “why”, remember that he/she is thinking “tell me more about this topic”. It could sound like this;
“Daddy, why are the ants crawling into that hole?”
“That is the hole into their house. They have a big house underneath that we can’t see.”
“Why?”
“Because that’s where their house is.”
“Why?” (Frustration may start here for some adults, because we feel we have answered the question directly. The toddler wants to know more about ants and what they do, but they don’t know how to formulate that question yet. They only know how to say WHY.)
“Well, ants live in big houses with lots of tunnels, and hundreds of doors to the tunnels. There are worker ants. There is a queen ant, and there are also little ant eggs. They work really hard to keep their tunnels clean, so they move sand all day. The babies need lots of food. And the queen ant tells everyone what to do.”
Many times, getting this much information will satisfy the toddler. It’s so much new stuff to think about, and it helps them get even more curious about ants. But it won’t lead to another “why” immediately.
The next step, if time permits, would be to read a book about ants, or put some food out for the ants and watch what happens. Basically do something that will continue to stimulate more learning and curiosity in the child about the topic.
Remember, asking “why” is a huge milestone that means your child is eager and ready to really learn about the world. Run with it! Just don’t get stuck in the mindset that “why” can only lead to one answer. It means “tell me even more, I am so interested in this!”
There’s another point to this answer I would love to reiterate, as some people have mentioned in their own answers and in the comments;
It can also be beneficial to throw the “why” back at your toddler in a curious, honest way yourself. If we continue with the ant story, when he asks why they build tunnels, you could say “Hmmm I wonder why also. Why do you think?” The answer truly doesn’t matter, because the exercise is about developing theory, and also practicing conversation skills. Even if he says “aliens probably built the tunnels for the ants!”, it’s ok to just nod and say “interesting! I wonder if that’s true!” …. There is so much going on for toddlers when they start practicing conversation, and all of it is worthwhile. Enjoy these years of part fantasy and part reality, because they will be gone before you know it!