Aha! A capital is a stash of money or the government headquarters of a state. Oh, a capitol is a building.

A state's capital is the primary city and usually the seat of the state government. The most important city is the capital city. The capital of New York is Albany, for example. Washington D.C. is the nation's capital:

All I knew was the capital of California, because Sacramento was the name of the street we lived on in Chinatown. (The Joy Luck Club)

Capital is also a financial word referring to, in a nutshell, the money a person or business has, not including debts. But wait, there's more! A capital letter starts a sentence or a proper noun, like the capital V for Virginia. It's also a crime that carries with it the severest penalty (life in prison or execution). Here are some examples:

Companies are encouraged to demonstrate that they can obtain other sources of funding, including private capital and state and local investment. (New York Times)

As early as 1682, William Penn’s colonial government experimented with incarceration as an alternative to corporal and capital punishment. (Newjack: Guarding Sing-Sing)

If you want to sound like a British spy, use capital to mean excellent, first rate, or really important:

In the realm of amateur fruit growing, on the other hand — a realm now daily widening — dwarf fruit trees are of capital importance. (F.A. Waugh)

A capitol (with an o) is a building that houses a government's legislative branch. When capitol is capitalized (ahem), it refers to the Capitol Building in Washington, D.C., that is home to U.S. Senators and Representatives. It's located on — where else? — Capitol Hill.

Remember that capitol with an o refers to a certain type of building that usually has a dome, then you'll know that in all other cases you want capital with an a.