Iron doesn't dissolve readily in water, although it will definitely rust more rapidly (as you've probably noticed from experience). Hydrochloric acid, however, can dissolve iron, and a more concentrated solution will dissolve it more rapidly. This simple experiment makes a great way to study reaction kinetics, but it does pose some possible hazards. First of all, it releases highly flammable hydrogen gas, so it should be performed under a fume hood. Moreover, hydrochloric acid is also a hazardous chemical if misused; it's especially important to avoid spilling it on skin or eyes. With these cautions in mind, you can dissolve iron using hydrochloric acid.