We love a handy trick that'll see us through when we're off on our adventures. Here's how to estimate the distance between you and a second point (for example, a river or lake in the distance) using just your thumb.
1. Find a stationary object near your desired destination whose size you can roughly estimate, like a barn.
2. Hold your arm out in front of you at arm's length, thumb pointing up like you're giving a thumbs up.
3. Close one eye and move your thumb so it covers the nearby object (point A).
4. Keeping your hand and head still, switch eyes so the other eye is open. The nearby object should appeared to have 'jumped' (point B).
5. Using the estimated distance of the nearby object, calculate how many of it would fit in the gap created when you switched eyes (so if the barn was 50 feet wide, and four of it would fit, the distance between points A and B. Multiply this by the magic number 10 (the ratio of the length of your arm to the distance between your eyes) and you've got the distance between the object and you!
Give this a try and see how you get on - did it help you estimate distance? Would you use this in real life?