From the brains behind brains on, this is the Moment of Um.

Moment of Um comes to you from APM Studios. I'm Howie Howerson.

Um... to you from APM Studios. I'm Howie Howerson. Um, my name is Howie, which is funny because my favorite

question is also how. I am always wondering, and I'm always wandering. Like yesterday, I wandered down to the

Science Museum. I checked out the bug room with its 463 different species of beetles. I know, because I

wondered how many.

So I counted them.

Down the hall from the bug room is my favorite part of the museum.

The dinosaurs.

I love all the fossils on display with those handy-dandy tags with lots of answers to my questions, like how old the fossils are.

But you know what that made me wonder?

How do scientists know how old all those dynos are?

My good friend Vernon was wondering the same thing, So let's see how an expert answers the question.

Hi, my name is Vernon from Elkins, West Virginia, and my question is, how do scientists know how old fossils are?

My name is Jamil Connor, and I am the associate curator of fossil reptiles at the Field Museum in

Chicago. I am a dinosaur paleobiologist, which is just a fancy way of saying, I am a paleontologist who focuses on dinosaurs.

Fossils only come from sedimentary rocks, and sedimentary rocks form in layers.

So we use something called the law of superposition, which basically means if you have this layer of rock, any rock on top of it is going to be

younger, and any rock below it is going to be older. So that's like step one, right? But when we really

want to get precise dates, what we're looking for in these sedimentary rocks are usually volcanic layers.

So if a volcano goes off and, you know, then you'll get this layer

of volcanic residue, right? And it will contain elements that we can use for radioisotopic dating.

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