Fill a cup measure halfway with water and put a pastry brush in it; this will be used for washing down the sides of the pan to prevent crystallization. In a heavy-duty 2-quart saucepan, stir the sugar, lemon juice, and 1/4 cup cold water. Brush down the sides of the pan with water to wash away any sugar crystals.
Bring to a boil over medium-high heat and cook, occasionally brushing down the sides of the pan, until the mixture starts to color around the edges, 5 to 8 minutes.
Gently swirl the pan once to even out the color and prevent the sugar from burning in isolated spots. Continue to cook until the sugar turns medium amber, about 30 seconds more. (Once the mixture begins to color, it will darken very quickly, so keep an eye on it.)
Photo: Scott Phillips