This isn't really a trick, it's just a cool way to close your lighter: Open your lighter, then place your thumb on the body of the lighter the side opposite the hinge and place your first finger on the open cap, as shown in the picture. Next slide your finger down the cap and onto the body. This should produce enough force to flip the cap closed and also produce a good trick. This trick usually only works with lighter which have been used a lot and the cap is a bit loose.

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note that the way some zippos are manufactured, this can be impossible (like once the zippo is opened the cap rests on the body instead of resting at an angle)