A Chinese Ball Button, often called a Chinese Knot, is a useful knot to know. It is often used as a button itself, or as the stopper for a button with a single large hole in the middle (like a Chinese coin). These buttons are often used with "frogging" or the sewn on loops of silken string that decorate an Asian style shirt or vest.
You can also use this knot as a stopper for anything with a small hole that you want to attach a string to.
Secure one end of the string. It's easier to tie the first part of this knot if the string is flat and the end is secured to something like fabric.
Make a loop (Loop A), crossing the string OVER the secured end.
Make a second loop (Loop B) that crosses over Loop A, but passes UNDER the secured end. (Click on picture if you need a better view.)
Make a third loop, but this time weave the thread through all the overlapping loops: over the first edge of Loop B, under the first edge of Loop A, and over the second edge of Loop B, and finally out under the second edge of Loop A. (Click on picture for a bigger illustration.)
If it's really important to have both strings come out of the knot together, you can weave the end of the string one more time, around the last string and in through the loop where the secured end of the string comes out.
Work the knot slowly tighter so all loops tighten up at the same rate. You can use this step to "sculpt" the shape of the button.
Trim off excess string, or tie the strings together in a simple knot to bring them together for use.
You can practice this with clothes line, or just get some thicker silken rope from your craft store.