First of all, you will need a rope, a wire cord or a ribbon, which you need to show to your audience and allow them to examine it.
Once the audience examines the rope, you just tell them that you can tie a knot in the centre of the rope, without releasing any of its two ends. It appears almost an impossible task to do. So, you can ask the audience to put a bet on it. You can set the amount the bet amount to $10, in order to get the bet easily. Most probably, you will be able to receive a bet on your trick.
Ask the member who placed the bet to hold the rope from its ends and create a knot in the centre. Also, you should tell him that he cannot release the ends after he grabs them. In addition to that, explain the audience that it is impossible to tie a knot until you will tell them the secret behind the trick.
Once you find that the member who placed the bet is tired of trying and accepts that it is impossible, you can take the rope back. Now remind the person that you only asked him not to release the rope while trying to knot in the centre. Also, tell him that you did not said anything about on how to grab the rope.
Fold your arms. Here, you should make sure that you do not cross your arms, or else, you will get stuck in the knot. Place your one arm on the top and the other on the bottom.
Now grab one end of the rope with your hand at the bottom and the other with the hand at the top. Once you grab the rope ends, pull your arms apart and the knot will automatically get tied in the middle. You have won the bet.