This playdough lasts for about 6-8 months if kept in an airtight container being played with once a week or so for a couple hours. I make a new batch about twice a year. Its fun for the kids to help and I let them choose what colors we do! I usually divide it into 4 pieces so they have 4 colors to play with.
2 c. boiling water
3 Tbsp. oil
food coloring- either paste or drops
2 1/2 cups of flour (and a little more for kneading)
1/2 c. salt
1 Tbsp alum
Mix dry ingredients in a mixing bowl. Add wet ingredients except food color. Mix as well as you can with a wooden spoon and then knead on the table until cooled slightly and very smooth- not sticky. Add food coloring while kneading. If you want to make several colors, divide the dough after kneading and then add the colors then.