Most of us at some point have ended up getting into a fight with someone who doesn't think that climate change is happening, or doesn't agree with evolution. And when that happens, one of their most common criticisms is always: "Yeah, but [insert rigorously tested idea here] is only a theory." Well, that's true. But in the science world, that word doesn't mean what you think it means. Joe from It's Okay to be Smart is here to explain why, and we think the video should be mandatory viewing for pretty much everyone at school.

The reason there's all this confusion is that words like "fact" and "theory" mean very different things in science than they do in everyday use. So let's set this straight once and for all so that we can clear up some of the misconceptions, or as Joe says in the video above: "If we're going to trust science together, the least we can do is speak the same language."  

To begin with, let's talk about fact. In science, facts are observations like, "Oh hey, it's bright outside." Once you start trying to explain that fact, for example by assuming the Sun must be out, you've created a hypothesis that can be tested. And then you just need to set up an experiment that can do that in a repeatable and controlled way. Well done guys, we just did a science!

Most facts have multiple possible explanations and multiple hypotheses to test. And gradually you can rule them out one by one until, sometimes, there's only one left. That doesn't mean that a hypothesis is right though, it's just a possible explanation. 

When enough hypotheses get the scientific tick of approval, they can be piled on top of each other to get a theory, and that's not a dirty word. In fact, it means that an idea has passed the toughest tests that science can throw at it, and has lived to tell the tale. "Theory" represents the highest amount of certainty we can have.

But then what's a law? And what happens to a theory when we get new evidence? Watch the video above to have it all ELI5. And never worry again about not knowing what to say back to all those climate deniers next time they start waving the "just a theory" argument in your face.