Place a large cooling rack on a baking sheet lined with parchment or wax paper. 

Insert a fork into the top of each apple.

Combine sugar, grenadine, and light corn syrup in a saucepan and bring to a simmer. Stir until the sugar is dissolved, brushing down any crystals on the sides of the pan with a pastry brush dipped in warm water.

When sugar is dissolved, bring to a boil until the syrup reaches 285° on a candy thermometer, the soft-crack stage.

Set the pan over - not in - boiling water. 

Working quickly, use the forks to dip the apples, one at a time, coating them evenly with the glaze.

Remove from glaze and twirl each apple so the extra syrup drips off. Set the apple on the cooling rack. 

When you have dipped all apples once, repeat the process.

Remove forks. Serve at room temperature. Best eaten within 24 hours.