This 9-minute video lesson discusses what a species is and isn't. It considers ligers, tiglons, mule, hinnies, and dogs. [Biology playlist: Lesson 61 of 71].

This 13-minute video lesson looks at the science of taxonomy and where humans fit into the tree of life. [Biology playlist: Lesson 62 of 71].

This 19-minute video lesson provides an overview of how the nephrons in the kidney filter blood and reabsorb water and other molecules. [Biology playlist: Lesson 59 of 71].

This 21-minute video lesson looks at the pulmonary system including the lungs, larynx, trachea, bronchi, bronchioles, alveoli and thoracic diaphragm. [Biology playlist: Lesson 37 of 71].

This 9-minute video lesson looks at tropomyosin and troponin and their role in regulating muscle contraction. How calcium ion concentration dictates whether a muscle is contracting or not. [Biology playlist: Lesson 48 of 71].

This 8-minute video lesson presents an overview of types of immune responses. It looks at the difference between innate and adaptive immunity and the differences between humoral adaptive immunity and cell-mediated adaptive immunity. [Biology playlist: Lesson 52 of 71].

This 20-minute video lesson discusses how variation can be introduced into a species. [Biology playlist: Lesson 7 of 71].

This 23-minute video provides an introduction to viruses. [Biology playlist: Lesson 19 of 71].

What cryptographic hash functions are and what properties are desired of them. Created by Zulfikar Ramzan.

A high-level explanation of digital signature schemes, which are a fundamental building block in many cryptographic protocols. Created by Zulfikar Ramzan.

An introduction to the mechanics of bitcoins and an overview of how transactions take place. Created by Zulfikar Ramzan.

An explanation of cryptographic proof-of-work protocols, which are used in various cryptographic applications and in bitcoin mining. Created by Zulfikar Ramzan.

The mechanisms by which the supply of bitcoins is controlled. Created by Zulfikar Ramzan.

A detailed explanation of what makes bitcoin transaction block chains secure. Created by Zulfikar Ramzan.

The mechanics of a bitcoin transaction block chain, which is a construct that is generated by bitcoin miners and functions as a global ledger for recording and validating bitcoins. Created by Zulfikar Ramzan.

The basic mechanics of a bitcoin transaction between two parties and what is included within a given bitcoin transaction record. Created by Zulfikar Ramzan.

Bit-zee is a project designed to inspire students to explore math and science. It was created from everyday items (a hair dryer, tap light, alarm clock, digital camera, and a universal remote). See the following link for help with the code https://sites.google.com/site/khanacademyprojects/home. Created by Karl Wendt.

Learn about the naval warfare between Britain and Germany that led to the sinking of the Lusitania. Created by Sal Khan.