Counting to three is so easy, a salamander can do it. Seriously. Lab experiments have shown that captive salamanders are able to distinguish between piles of two fruit flies and piles of three. If you're not impressed, we understand. A human being who'd never taken a single math class would have no trouble doing the same thing. Some single-digit numbers like one, two and three are so small that our minds can recognize their value without even needing to count. Put a tray of three cookies in front of your average adult and he or she will immediately (and intuitively) know how many there are. No fingers or calculators required!
Yet as numbers grow bigger, our ability to comprehend their values starts to break down. The word "billion" gets tossed around a lot by economists and politicians, but it's hard to appreciate just how large that sum is. One billion seconds is equal to 31 years, 251 days, 13 hours, 34 minutes, and 54.7843 seconds (not counting leap days and leap seconds)