Therefore, the Pi bond is less stable because it's higher in energy, and therefore it's more reactive. Now, whenever we input energy to break our Pi bond or to break our double bond, our pi bonds break first. So let's look at one more important detail about our double bonds. So, this is a single bond. This is an ethylene molecule. So notice that in an ethylene molecule, we have one Sigma bond, and the Sigma bond is able to rotate.