Heat the oven to 350°F and grease a 9 x 13-inch baking dish. Prepare the cake mix with the eggs, vegetable oil, and water according to the directions on the package, then pour the batter into the prepared pan.

Bake the cake for 30–35 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool for about 15–20 minutes before adding the gelatin.

Use the handle of a wooden spoon or a large straw to poke holes about 1 inch apart across the cake. Make the holes reach most of the way through the cake without breaking through the bottom.

Whisk the strawberry gelatin powder with 1 cup boiling water until completely dissolved. Stir in 1/2 cup cold water, then slowly pour the mixture evenly over the cake so it flows into the holes.

Refrigerate the cake for at least 1 hour, preferably 2 hours, so the gelatin can set and the cake can become thoroughly chilled. Spread the thawed whipped topping over the chilled cake and garnish with sliced strawberries.