Each of your eyes has a blind spot that corresponds to where the optic nerve attaches to your eyeball. So why don't we see some kind of black or gray patch where the blind spot is? Because your brain "hides" the blind spot! Click on the picture to see how to fool your brain into revealing where your blind spot is. There's also a good demonstration here.

The Sliding Sheet Bend is my favorite knot. Peter Suber shows how to create it at his web page. This amazing knot is very easy to tie, it slides to adjust taughtness, it locks, and it "explodes" as Peter puts it.

The Clove Hitch biting a Bight should be the next knot you know. It's not as fun as the Sliding Sheet Bend, but it's very easy to remember, and very easy to tie. I use this knot instead of the Bowline all the time. This is the first knot illustrated at Peter Suber's web page.

How do some trees grow to arbitrary height and grow each branch such that they're always distributed to receive the most sunlight? The Golden Ratio (or Golden Section), represented by Phi, is pretty spectacular. More general than the Fibonacci sequence, and very visible in nature. The Golden Ratio can be seen in the Nautilus Shell, the location of branches on a tree, the packing of seed heads, and the petals on flowers. Dr. Ron Knott has an excellent introduction at his site. Visit his site, you'll be glad you did!