Although there are a few ideas about what made the dinosaurs extinct, I think the massive asteroid impact theory has the most evidence so far. This suggests that a HUGE asteroid hit the earth, carving a very large hole that we can still see today in Mexico at a place called Chicxulub http://en.wikipedia.org/wiki/Chicxulub_crater . This impact filled the air with dust that for years reflected the sun’s rays away from the ground and plunged Earth into a long period of very cold weather. The dinosaurs couldnt cope with this cold weather and were out-competed by the fur covered mammals and so they went extinct.
On the other hand… we now believe that the birds are the direct descendants of the dinosaurs, which would actually make them dinosaurs and not a special group all on their own! So… we could very well have dinosaurs in our back garden happily taking a bath in an old flower pot.
Hello! I agree with Andrew – it was a realllllllly big asteroid that hit the Earth that wiped them out.
In the past year, some scientists have found evidence that the number of some kinds of dinosaurs (because remember, there were lots of different dinosaurs – ranging from small herbivores to massive carnivores!) were already starting to decrease.
This means that the big asteroid sort of just helped them along (well I guess maybe ‘help’ isn’t the best word – since it was making them go extinct!), since they were likely going to die out anyway.
I think it would be really cool to be a palaentologist, and get to go searching for dinosaur fossils to try and figure all of this stuff out!