You're right, nitrogen is generally considered a non-combustible gas. However, magnesium's reaction with nitrogen isn't typical combustion. Here's why magnesium burns in nitrogen:

* High Reactivity: Magnesium is a highly reactive metal. It readily reacts with many elements, including nitrogen.

* Formation of Magnesium Nitride: When heated, magnesium reacts directly with nitrogen gas to form magnesium nitride (Mg3N2). This reaction is exothermic, meaning it releases heat and sustains the burning process.

* Not Combustion in the Traditional Sense: The reaction between magnesium and nitrogen is not typical combustion, which involves rapid reaction with oxygen to produce heat and light. Instead, it's a direct combination reaction where magnesium and nitrogen combine to form a new compound.

Here's the equation for the reaction:

3Mg(s) + N2(g) → Mg3N2(s)

In essence:

Magnesium's high reactivity allows it to combine with nitrogen, forming magnesium nitride, which is a stable compound. This reaction generates heat and light, giving the impression of burning, even though it's not a traditional combustion process.