Question: How do you guys handle the constant "but why" phase?

My 4yo has entered a new level of "why" that I didn't think was possible. Like I'll explain something and he'll ask why like 15 times in a row until we're talking about the sun exploding or whatever. It's exhausting but also kind of hilarious? Is this normal or am I doing something wrong? How long does this phase actually last because I need to know if I should start taking notes or if my brain will just break eventually.