Why do the light bulbs in a combination circuit have different brightness?

Each light bulb will have their own resistance. So when they are connected in series the light bulb that first receives the current will shine the brightest, the second light build will receive the current - the resistance of the first light bulb, hence, it will shine less than the first light bulb. The third light bulb will receive the circuits current - the resistance of the first and second light bulb so it will shine the least. The pattern will repeat itself for as many light bulbs in the combination.