Simple hemp bracelet that can be pulled apart easily when you need some string, expanding to 6-7 times its length.
Step 1: The Materials
You'll need some string (either hemp or something else) that's around 6-7x the circumference of your wrist (that might vary depending on the thickness of the string, increase it if you're worried), a small bead, and a pair of scissors or a knife.
Step 2: Start Knotting
I apologize in advance for out of focus images. My camera's auto-focus is broken. I'm using yarn in these pictures to make it easier to see the knot.
Make a loop near one end of the string. Leave 3-4 inches (8-10cm) free at the end. Put your fingers through the loop and pinch the long part of the string. Pull a new loop (horizontal piece in second picture) through your first loop.
Tighten the first loop you made by pulling on the new loop. Make sure you don't pull the free end (orange) through - you want to get a new loop like you see in the third picture.
Repeat the above steps by reaching through the loop and pulling a new loop through it, then tightening the knot. It's just like the first chain you'd make crocheting.
Step 3: Close the Bracelet
When you've knotted enough length that the bracelet wraps comfortably around your wrist, it's time to tie it on. Normally you could just tie the two ends of the string together, but we want the bracelet to come undone easily without untying anything, so we'll need a different way to connect the ends.
Go back to the opposite end of the string (where you started) and make a new knot, as if you were starting the knotting process in the opposite direction. Tighten the knot but don't pull a second loop through, just leave the one loop you just made.
This part is a little awkward, you might need a friend to help hold everything. Put the bracelet around your wrist. Put the loop at the end of the bracelet through the loop at the beginning (the one you just made). Tighten the loop at the beginning (red in pictures). It helps to hold it like in the second picture (the part I'm holding corresponds to green in the pictures).
Step 4: Bead "ripcord"
Tie two more knots on the end of your bracelet (green).
Take the free string from the new loop you made in step 3 (red in the pictures) and feed it through the bead. Feed the string through the last loop of your bracelet (green in the pictures).
Tighten the green loop by pulling on the green string (picture 2). Then pull on the red string. You might have to pull on each of them a few more times to make sure they both are tight (picture 3).
Optional: Tie a regular old knot of your choice on the green string as insurance that when you pull on the bead the correct string (red) comes free.
Step 5: Done!
Wet the bracelet to tighten the knots. Trim off the excess strings, leaving about an 1/8 inch (.3 centimeter) stubs.
When danger strikes and you need string immediately, pull on the bead until it comes off (releasing the red string from the pictures) then pull on the other string (green) until the bracelet comes off. Keep pulling and the whole thing will unravel, giving you about 35 inches (90 cm) of string whenever you need it. Because of the way it's tied around your wrist, pulling on the bracelet itself won't do anything, so it shouldn't fall off during normal wear. If it does, leave bigger stubs next time (and do the optional part of step 4: tie a normal knot on the green string before trimming off the excess).