Algorithm question

In the bin-packing problem, we are given n items with weights w1, w2, ellipsis, wn, respectively. Our goal is to find the smallest number of bins that will hold the n objects, where each bin has capacity of at most one kilogram. The first-fit heuristic considers the objects in the order in which they are given. For each object, place it into first bin that has room for it. If no such bin exists, start a new bin. Prove that this heuristic uses most twice as many bins as the optimal solution