Burning sugar

You need lumps of sugar, some cigarette ashes and matches or a lighter.

Not that this trick won´t work with granulated real sugar, but it is more impressive with a lump!
Normally, if you try to light sugar with a flame, it may bubble and melt, but it won´t actually catch fire. Unless you know how!

Challenge your friends to light a cube of sugar. While they are attempting to get one of the sugar cubes to light, secretly get some cigarette ash on your finger tips. Try to get as much as you can.
Have an one of your friends select a sugar cube, and as you pick it up transfer the ash to the sugar cube. Now, hold a flame to the portion of the cube with the ash on it, and it will light easilly!