Quick anatomy lesson: the distance between your eyes is one-tenth the length of your arm. This is important, because it helps you estimate the distance between yourself and any object of reasonably known size to a rather accurate degree.
Let's begin this with a confession: I can't estimate distances worth a darn. 100 ft, 300 ft, 20 m, I can't really do it. Thankfully with this trick, we can get pretty accurate distance estimates with only the tiniest bit of math, a general idea of the size of the object you're estimating distance from, and an arm.
Basically, all you need to do to estimate a distance using this classic method is hold your arm out and stare at your thumb with an eye closed. Silly, yes, but it makes sense, because you'll switch which eye you look through and sort out how far the object appears to have 'jumped' to the side:
How far did it move? (Be sure to sight the same edge of your thumb when you switch eyes.) Let's say it jumped about five times the width of the barn, or about 500 feet. Now multiply that figure by the handy constant 10 (the ratio of the length of your arm to the distance between your eyes), and you get the distance between you and the barn — 5,000 feet, or about one mile.
Easy enough, and from a few quick tests with known distances, it appears to be accurate enough for ballpark figures.
The Old Farmer's Almanac has been around for as long as we recall, but it is still a great source for MacGyver-style tricks, even if this particular one requires no pens, matchboxes, or staples. Take a look at the link for more details on this estimation 'rule' and others and be sure to stop in the comments to share your oldie-but-goodie sort of tricks. Thanks, TechTalk WRLR 98.3FM!