Unless you are a teenager or were recently educated, the answer might be, not much. Half of what you think you know may not be true.
It was a common fact when we were kids that dinosaurs were cold-blooded. Evidence today indicates that at least the larger dinosaurs were likely warm-blooded. Many believe they had feathers. Bet you were never taught that. There are nine planets in our solar system, right? Wrong. Pluto is no longer considered a planet and has been downgraded to a dwarf planet. Astronauts are weightless as they orbit the earth. Everyone knows that. Not true either. It is true that they experience near zero g-forces much like a person in a rapidly declining airplane, but, while orbiting the earth, they experience about 90% of the gravitation pull as we do on the surface of the earth. Napoleon was short. It is where we get the term, "Napoleon complex." I've know many with a Napoleon complex. As it turns out, he was a little taller than the average European. He just appeared shorter when surrounded by his taller guards, and they may have been an issue with unit conversion. The Great Wall of China is so vast that one can see it from space. If you have ever been there, you would know that it is not any wider than a typical road. Can you see roads from space with the unaided eye? No. And, George Washington didn't have wooden teeth. Most of us know by now that he didn't chop down the cherry tree either.
There are terms bandied about today such as the half-life of knowledge or the half-life of facts. According to Wikipedia: "The half-life of knowledge or half-life of facts is the amount of time that has to elapse before half of the knowledge or facts in a particular area is superseded or shown to be untrue. These coined terms belong to the field of quantitative analysis of science known as scientometrics.
A book was recently published called, "The Half-Life of Facts" by Sam Arbesman (sorry Tom). I haven't read the book, but I have read interviews and watched his TED presentation. Arbesman makes several astute points. One is that facts have a shelf life. He compares it to the half-life of radioactive material. At first glance, it might seem that he is trying to force his idea of facts into a scientific formula. After all, some facts remain with us over time. I never thought about radioactive decay in the matter which he described it. It is rather stupid of me because I had a course in nuclear chemistry decades ago. We don't know when an individual atom will decay. It might be in the next fraction of a second or it might be millions of years from now. Cumulatively and statistically, we can accurately estimate the half-life of radioactive material, however. We can't apply the idea to individual facts, but, we can apply it to facts taken as a group. It makes sense.
We can actually measure the rate of scientific progress. One way is to measure the number of published studies. Price estimated that scientific knowledge was doubling every 10 to 15 years. Recent studies indicate that a similar trend continues. But, some studies can't be replicated, some are just wrong, some are lost, and many get superseded. One estimate is that 30% of discoveries aren't discoveries at all. My favorite is cold-fusion. A study was published about the success of cold-fusion and soon universities around the world were replicating the experiment--only they weren't. When cooler heads prevailed, cold-fusion had failed. It couldn't be replicated.
Not all sciences progress at the same rate. For example, there is not much new under the sun when it comes to mathematics. New sciences come along regularly. Climate change is one example. Progress can be so regular in some sciences that we can use this knowledge of growth rates to project when technology will become available. It has been said that the military used the growth in scientific knowledge to predict when the US would have the technology to put a man on the moon. They were pretty accurate.
The half-life of scientific knowledge can be estimated by how often a study is cited. Turns out the half-life of scientific knowledge is about 45 years, according to Arbesman. Half of what we know is not useful after 45 years. We can't say which facts, but we can say with reasonable certainty that a large chunk of what we think we know today will be proved to be untrue.
As I said, not all sciences progress the same. Some change more than others. Social sciences depend on input from people and are more difficult to measure. The half-life of knowledge in psychology is estimated at five years. In other words, half the knowledge we had five years ago has either been found to be false or has been superseded. I would argue that the half-life of climate change knowledge is even less, given the rate that it changes.
The half-life of facts on the internet may be the shortest of them all.
My father was a simple man, but he was also wise. He told me once that "An intelligent man is never sure." I took that lesson to heart. Whether or not I am intelligent is not for me to judge, but that is how I approach life. I admit that I don't know everything and I am open to the idea that I can be wrong and what I know today may be meaningless tomorrow. I will never say the science is settled.