By rubbing it with sand paper which removes the oxide layer of magnesium
Because the oxyen is present on it so it can take more time to burn but if we clean with sand paper before burning
Becaz it gets reacted with oxygen present in the air and becomes magnesium oxide
Magnesium is very reactive element. When stored , it react with oxygen and a layer of magnesium oxide on its surface and protect magnesium from further reaction with oxygen. Thus magnesium ribbon is cleaned before burning in air to remove this layer.