1/4 c. Key lime juice
1 container white frosting.
Add about 1/4 c. Key lime juice to 1 container white frosting. Mix well with fork. Will be slightly runny on cake, but hardens quickly in fridge.
Add lime juice 1 T. at a time, to taste. We like things *really* limey, but you might like a more subtle lime flavor.
Use lime juice instead of water in white cake mix, top with lime frosting, and you've got a refreshing cake.
Any citrus juice will work