You could fill a room with dumbbells, or you can get up to 55 pairs at a fraction of the cost with 1 set of PowerBlocks. Each PowerBlock has color coded side rails on the weight plates that correspond to an easy reference color coded weight chart on the handle. The weight plates are nested, which means the handle fits inside weight plate 1, which fits inside weight plate 2, etc.
To change the weight in just seconds, slide the selector pin into the blocks at the weight you want. Then reach inside the "core" and grab the handle. When you lift the handle, the weight you selected is lifted along with the handle.