Fill the gelatin with fruit juice of your choice in a saucepan and let it swell for about 10 minutes.
In the meantime, squeeze the lemon and strain the juice through a very fine sieve.
Add the lemon juice and sugar to the gelatin-juice mixture in the saucepan. Heat the mixture, stirring constantly, until each ingredient is dissolved and completely combined: Be careful that the liquid does not begin to boil under any circumstances!
Pour the finished mixture into silicone molds and place in the refrigerator to set for at least 2 hours.
Remove the finished fruit jellies from the mold and enjoy.