# 11 Khan Academy Lessons That Will Change How You Understand The Universe — In Three Hours

Khan Academy is a site that hosts thousands of lectures in topics from physics to finance. They’re divvied up into videos that are anywhere from 3 to 15 minutes long.

It’s a far cry from a typical classroom experience, but so far Khan Academy has been a smash hit. The site gets around 39 million pageviews and 3.5 million unique users each month, Khan said at the Web 2.0 Summit in San Francisco last month.

Founder Salman Khan started the site to build an education system where students work at their own pace. Students are encouraged to continue working on the same topic until they master it and move on.

Khan just finished raising \$5 million for his online education site — and he’s using it to hire new faculty and build a physical school based on the Khan Academy model.

Until then, we’ve assembled a list of some of the coolest lessons available on the site.

### The Golden Ratio explained

Two quantities are said to be in a 'golden ratio' if the ratio of the sum of the quantities to the larger quantity is equal to the ratio of the larger quantity to the smaller one.

Complicated, right? Except the Golden Ratio shows up everywhere. You'll find objects in the golden ration in nature, biology and even financial markets. Like the Fibonacci Sequence, it's a mathematical phenomenon that appears regularly enough that it demands a definition.

### How to always win a card game

Ever wanted to destroy your friends in a card game?

Your best bet it to start brushing up on basic probability. Using some pretty simple maths, Khan explains how to determine your chances of getting the card you need in a card game like Blackjack.

### Revealing the tiniest parts of the universe

The universe is absolutely massive. But there's also a lot of really interesting stuff happening at the small scale.

In fact, some things are so small that we can't even understand or visualise them. In this video, Khan goes through some of the smallest units of distance and what happens at those tiny scales.

### How economic bubbles work

When the housing bubble popped, it destroyed fortunes and left thousands of people without jobs.

Khan explains how financial professionals and just about everyone else lost money when trillions of dollars evaporated overnight at the start of the 2008 recession.

### Here's how Black Holes work

Getting sucked into a black hole sounds like it would hurt. But let's be serious -- it would be a really cool way to die.

Khan explains the maths behind black holes and how they work in this video.

### This is how heart disease kill you

Heart disease is the number-one cause of death in the United States.

For something so menacing, it's hard to imagine it all boils down to some simple fluid dynamics. But that's basically what happens, and Khan explains how your heart runs into problems when plaque starts building up in your arteries.

### Here's how the earth was born from a supernova

The Earth is a the byproduct of a local supernova -- a gigantic stellar explosion.

Khan explains the whole process of how the earth started as a result of that supernova and how it turned into the planet we inhabit today.

### This is how gravity works

When you stand next to a really dense object, gravity pulls you closer to it.

You won't be able to see it, but gravity is one of the fundamental forces in the universe. Khan explains how it works and why large, dense objects pull other smaller objects closer.

### This is what happens when two GALAXIES collide

Galaxies are also attracted to each other by the force of gravity.

They can also smash into each other. Our own galaxy, the Milky Way, and another local galaxy named Andromeda are actually set to run into each other -- though it won't happen for some time.

But you can bet when it happens it will look awesome. (Or we'll all die.)

### The Richter Scale is not as complex as you think

There have been quite a few notable earthquakes in recent months -- even one reaching as far as New York.

So now's probably a good time to explain just how powerful those earthquakes are, and how the scale that measures them works. The Richter Scale is actually a logarithmic scale -- meaning an increase of one point in magnitude means the earthquake is 10 times stronger.

### This is where it all comes together

In Sal Khan's own words: 'If this doesn't blow your mind, you have no emotion.'

Euler's equation -- and the mathematics behind it -- are a fundamental component of a field of mathematics called complex analysis. We use complex analysis every day to tackle the most challenging and interesting problems. And it all starts with one formula.