eyes
...they say are the window to your soul, so make sure they are as healthy as they could possibly be.
those who wear glasses and contact lenses will have check-ups every now and then, but those who don't should go and get a test at an opticians. Eye-tests can pick up problems like glaucoma and cataracts as well as problems that don't even relate to the eyes, such as diabetes. For all sorts of information about eyes go to www.rnib.org.uk.
for information on Glaucoma try the International Glaucoma Association.