The underlying idea is that a dollar today is worth more to you than a dollar tomorrow (all else equal).

The NPV is how much a thing is worth/costs now after taking that into account and discounting future cashflows appropriately.

At large enough size, commission/trading costs are not necessarily a significant factor--individuals are just not great at stock picking and timing the market.

Absolutely true. However, individuals don't necessarily have enough money in their brokerage accounts to hit the point where trading costs become insignificant, especially if they're trying to diversify.

At those larger size, the bid/offer spread becomes much more significant, and is usually a bigger cost than the commission.

Let's say I sell you an option to buy (from me) a share of TWTR at $100 at any time through the end of January. I have just sold you a call option.

If I already own a share of TWTR that is not tied up in another contract, the call is considered covered (meaning I would not have to go out and find a share to buy if you exercised the option).

More: https://en.wikipedia.org/wiki/Covered_call

They work, but they are "mailto:" links.

It is not denominated in USD. It's a currency, and like any other currency you can exchange it for others, including USD, which is why you often see exchange spot prices in USD.

The exchange rate is denominated in USD. It's the BTC/USD rate... What's in the denominator

One exchange rate is BTC - USD. That does not make the currency itself USD denominated. Bitcoin is almost always denominated in BTC these days. There are other bitcoin denominations, like the Satoshi [1], but these are rarely used as BTC has not appreciated enough in value for them to be useful. You seem to be confused about what denomination means in the context of currency, see [2].

1. https://en.bitcoin.it/wiki/FAQ#What_do_I_call_the_various_de... 2. http://en.wikipedia.org/wiki/Denomination_(currency)

On the contrary: you have confused 'denomination' and 'denominated': http://www.thefreedictionary.com/denominated

It's obviously a coin made out of a bit of metal.

I've used it the correct way only to have people try to correct it with the (traditionally) wrong pronunciation.

One of my favorite tweets is by Ed Felten:

"If n > 2, then a^n + b^n does not equal c^n. I have discovered a truly marvellous proof of this, which this tweet is too small to contain."[1]

[1] https://twitter.com/edfelten/statuses/1806515554

It's not about the cost of college, at least here. Places like Stanford and Princeton have generous enough financial aid programs that if you can't pay for it, they will (presuming you're accepted).

True. Having applied to one of the two I'd like to point out that this is (with very few exceptions) only true for domestic students.

Arbitrageurs, not speculators.