The Dow...

The Dow is based on the stock price of 30 arbitrarily picked companies.

For comparison, the SP500 is based on the market capitalization(the size of the company) of our 500 largest companies.

Using the Dow's price weighted logic Berkshire Hathaway would be considered over 2,000 times larger than Apple, when in fact Apple is about 1.5 times larger than Berkshire Hathaway. That means you would be overestimating the size of Berkshire Hathaway to be 3,000 times larger than it is... or underestimating the size of Apple by 3,000 times... or worse, some combination of the two.

The Dow uses this funky math because it was created in the late 1800s when it was hard to find useful data on 30 companies, yet alone thousands, and we didn't have computers so the math needed be simple. Result: something that points in the right general direction most of the time, but it is by no means an accurate gauge for measuring the US stock market. Analogy: The Dow is like a rusty compass that occasionally gets stuck pointing south, and the S&P 500 is like GPS.

Yes, celebrate the Dow hitting 20,000. While we are celebrating made up numbers hitting meaningless milestones I would like to celebrate Pikachu's birthday using the Mayan Calendar. Does anyone know what day that would be?