You've said something, and your kids won't let it go. "What did you say?" they ask. "What did you say?" You repeat yourself to no avail. They get frustrated, and so do you. Are they deaf? Aside from the obvious answer, which is yes. No. They're not. They're just not understanding the concept behind what you have said.
Sometimes they'll give you a clue, and ask specifically what a particular word means. But even then frustration can occur. It frequently goes like this:
"Mama, why is that guy on the roof?"
"He's cleaning the gutters."
"What's a gutters?"
"They're like opened pipes that catch the rain and divert it to the ground safely."
"They make a path for the water to go through."
"When the rain comes down, it slides down the roof, and collects in the gutters, the open pipes, and then flows down to the ground."
"...I don't know."
"What don't you know? Why don't you know that?"
"I don't know."
"But why, mommy? And also, what does open pipe mean?"
"Well, that's not really what they are at all, it's just--Ugh, I can't."
So, what do you do?
For the first scenario, try to find the question behind the question. It's not that they didn't hear you (at least not after the second time). So what is it they are trying to understand. Try to answer the question they didn't ask. Not changing your answer to find something acceptable to them (which I sometimes do, by accident), but changing your words so that they might understand them more easily. They'll appreciate consistent content but more easily digestible explanations.
For the second scenario, do not give up.
They will absolutely ask you questions until you are dizzy and you don't know what you are talking about. Don't worry about it. We can't know everything, and I've found that my kids' crazy questions can open up new knowledge for me, or change my way of thinking, even, by forcing me to examine the reasons behind why I think certain things work the way they do.
Keep answering them. It lets them know their questions are important, that they are important. It keeps their curiosity alive. When you get to a place where you really don't know the answer, look it up. If you can't at that moment, assure your kids you will, and follow up with them.
Don't brush these learning opportunities aside. Yes, they can be annoying, but the longterm outcomes are worth it.