Lemon-Lime Watermelon Shots

Using a spoon, scoop out the pulp of the lemons, leaving the peel intact. In a small saucepan, heat soda and lemon juice until warm. Whisk in JELL-O packet and cook until powder is completely dissolved. Remove pan from heat and whisk in vodka.. Place lemon halves into wells in a muffin tin and fill with Jell-O mixture. Refrigerate until set, at least 3 hours and up to overnight.  Slice lemons in half and serve.