1. The argument for extinction by asteroid
The Earth's crust is made up of layers of rock.
The deeper the layer, the older it was created.

2. Fossils of dinosaurs are only found in layers of rock formed between about 250 and 65 million years ago.

3. About 65 million years ago an asteroid collided with the earth.
The collision caused a 180 km wide crater, near what is now Chicxulub in Mexico.

4. A small layer of dust formed about 65 millions of years ago and has been found in many places around the earth.
We know that dust in the atmosphere can effect the climate.

5. This dust contains large amounts of the element iridium.
Iridium is usually extemely rare in the earth's crust.
Iridium is much more common in asteroids.

6. So...
 The asteroid must have caused the dust layer.
 This dust in the atmosphere would have altered the Earth's climate.
 There are no dinosaur fossils in layers of rock above the iridium layer.
 The dinosaurs must have been unable to survive the climate change caused by the asteroid impact.