Advice on toddlers asking "why" all of the time

So I have 3.5 year old twin boys, and as of lately, we have apparently been going through a "why" phase. All day, seemingly every day, they come up to me and ask "why?" And I usually give them a reasonable response and answer their "why" question, but it feels like even when I answer them, they just keep asking why, as if they weren't listening. I repeat my answer, and they just keep asking why. I've never dealt with this before, and honestly, it's a bit annoying/frustrating when all they do is ask me "why" over and over. I know asking questions and asking "why" is a part of them learning and understanding, but how do I go about this in a good way? They don't even seem to actually care about the answer, just on pushing the boundary.