I found this on my favorite site – Allrecipes.com: http://allrecipes.com/Recipe/Idas-Lemon-Mousse/Detail.aspx
1 3/4 cups boiling water
1 large package lemon flavored gelatin powder
3/4 cup cold water
1 (12 fluid ounce) can frozen lemonade concentrate, thawed
1 (12 ounce) container frozen whipped topping, thawed
Dissolve gelatin in boiling water, then stir in cold water and lemonade. Chill in refrigerator until thick but not set, about 30 minutes.
In large bowl beat together chilled lemonade mixture and whipped topping until completely incorporated, smooth and pale yellow (about 10 minutes). Pour into a large mold or individual dishes and chill until set.