Cilantro makes a great substitute for watercress in sesame noodles. In Utah and Maryland we found watercress relatively easy to get — pick some up at the supermarket, or in Utah, stop along one of the mountain streams and pick it (not in the National Forest or National Park lands, of course). Down here in Texas, watercress isn’t as easy to come by.
Cilantro, on the other hand, is readily available.
Cilantro is also known as Chinese parsley, and as coriander. The herb is originally known from Greece, but is now grown almost world wide, and is a staple in Oriental and Southwest cuisines.
In your social studies classes, are you talking about spices or the spice trade? The Louise M. Darling Biomedical Library at UCLA has some excellent images and solid information on spices, their use and importance.