• Why do I add corn syrup?
Corn syrup acts as an "interfering agent" in this and many other candy recipes. It contains long chains of glucose molecules that tend to keep the sucrose molecules in the candy syrup from crystallizing.

In fudge, the addition of "interfering agents" can be a double-edged sword: you want crystals to form, but not until the cooked mixture has cooled down to a certain degree.

• Why do I add vanilla?
Vanilla is often added to chocolate candies or other chocolate recipes because it complements and accents the flavor of chocolate.

• Why do I need to stop stirring after the syrup begins to boil?
At this point, you have dissolved the crystal structure of the sugar. Stirring or other agitation is one of the many factors that can encourage the fructose and glucose molecules in your syrup to rejoin and form sucrose—crystals of table sugar.

• Why do I wash down the sides of the pan?
The sugar crystals are dissolved at this point in the process. But a single seed crystal of sugar clinging to the side of the pan might fall in and encourage recrystallization.

• Why does the fudge need to cool for such a long time?
The key to a smooth and creamy texture is a fudge that's full of thousands of tiny sugar crystals. Heating the syrup to a high temperature and then allowing it to cool, undisturbed, produces a supersaturated solution—this means that the solution contains more sugar molecules than would normally be possible at room temperature. A supersaturated solution is highly unstable, and any agitation will cause crystallization to occur throughout the solution. If fudge is stirred while it's still hot, fewer crystals form, and they grow larger as the syrup cools, resulting in a coarse, grainy candy.

• Why is it so important to keep stirring until the fudge "sets"?
Stirring helps control the size of the sugar crystals that form—it keeps them from growing too large, which would produce gritty fudge.

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