Sal reflects on the amazing speed and precision with which DNA replication takes place.

How a supply shock can cause prices to rise and the economy to stagnate. Created by Sal Khan.

How the Supreme Court uses the principal of stare decisis when incorporating precedent into decisions.

Brown v. Board of Education and how some states tried to get around it.

Thinking about the coefficients of static and kinetic friction. Created by Sal Khan.

Why the value per share does not really get diluted when more shares are issued in a secondary offering. Created by Sal Khan.

The strategy of the Civil War for the Confederacy (the South) was to outlast the political will of the United States (the North) to continue the fighting the war by demonstrating that the war would be long and costly. The Confederacy also hoped to entice European powers such as the United Kingdom to assist them against the United States in order to protect their cotton supply. The strategy for the United States was to surround the territory of the South in the Anaconda Plan, blockading the Atlantic Ocean and controlling the Mississippi, to keep goods from going into or out of the South and forcing them to surrender.

The law of demand states that quantity demanded increases when price decreases, but why? Two reasons why the demand curve slopes downward are the substitution effect and the income effect. The income effect states that when the price of a good decreases, it is as if the buyer of the good's income went up. The substitution effect states that when the price of a good decreases, consumers will substitute away from goods that are relatively more expensive to the cheaper good. Learn about the role of the income effect and the substitution effect on the shape of the demand curve in this video.

An introduction to the Sunni and Shia schism that forms in Islam after the death of Muhammed.

As Ali becomes caliph, the Ummayads under the leadership of Muawiya refuse to recognize him, sparking the first Muslim Civil War. The increased division leads eventually to the Tragedy at Karbala which is a defining event for Shia Muslims.