Peel, core and chop apples. Wash, dry, trim and chop rhubarb. Put fruit in large saucepan with water. Heat gently; simmer until tender, about 20 minutes, stirring all the time. Check often to see no sticking occurs. When tender, add the sugar; stirring until dissolved. Boil rapidly at first, stirring, then boil more carefully until set (about 20 minutes), stirring frequently. Cover pot as it cooks down. Ladle into hot sterilized jars leaving 1/4-inch headspace. Wipe rims clean, put lids in place and tighten down. Invert for a few minutes, then return to upright position and let cool completely. Check the seals; label and store in a cool dry place. Excellent with pork chops.