comment emboîter les fourchettes

Here you have to block the matchstick inside the prongs of 2 intertwined forks. Place them carefully on the edge of the  glass feeling for the point where the whole thing is balanced. With a bit of practice you can do it very quickly and always be very impressing!
Les explications de Buzzy:
Here I have represented the fulcrum point of the 2 forks by G1 and by G2. These being exactly the same, the fulcrum point of the whole is in the middle of G1G2 that is G. This point is precisely on the vertical from the point of contact P of the match on the glass. This last condition is necessary for the balance of the system. As G is situated lower than P so balance is stable that is you can move it a bit without it falling down...
et oui, c'est moi !
...Thus once you've filled your glass and burned the small bit of match which overlaps on the inside of the glass, stability will enable you to perform the highlight of your show like in this photo