Capital & Capitol

With an A – refers to uppercase letters, an accumulation of available wealth (with some indecipherable Econ 101 definition, I'm sure), or a city which is the seat of a country's or state's government.

With an O – the building itself used by a country's or state's legislative officials.

And don't forget, Capitol must always be capitalized.

"I'm going to take all my capital out of the bank and spend it on a trip to our nation's capital, where I'll write an angrily misspelled protest sign in capital letters and hold it up in front of the Capitol."