31.   Over medium heat, gradually whisk in the butter until the sauce has thickened.

32.   Raise heat to medium and cook, stirring occasionally with a wooden spoon, until sauce thickens.

33.   Pour in the milk mixture and stir-cook until the sauce thickens.

34.   Return the mixture to the saucepan and cook over moderately low heat, stirring with a wooden spoon, until the sauce visibly thickens and coats the spoon.

35.   Return the pan to the stove and cook over medium-high heat, stirring constantly, until the sauce thickens and becomes bubbly.

36.   Sauce will thicken as it cools.

37.   Sauce should thicken enough to lightly coat a spoon.

38.   Sauce will thicken as you stir.

39.   Sauce thickens slightly as it cooks, and foam will form on top, but it will subside.

40.   Sauce will thicken more as it cools.