21.   Continue to cook until the mixture thickens to the consistency of custard.

22.   Cook until mixture thickens, stirring constantly.

23.   Cook, stirring, over medium heat until the mixture thickens.

24.   Cook over medium heat, stirring occasionally, until mixture thickens.

25.   Cook over medium-low heat until the sugar melts and the mixture thickens.

26.   Cook over medium heat, stirring constantly, until mixture thickens and boils.

27.   Continue cooking, stirring often, until mixture thickens.

28.   Cook and stir over medium heat until mixture thickens and begins to bubble.

29.   Cook and stir over medium heat until mixture thickens slightly and bubbles just around edge.

30.   Cook over medium heat until the mixture thickens.