Tie the string or yarn to the pencil or butter knife. Set across the top of the jar so the string dangles without touching the sides or bottom. Make sure the string gets close to the bottom.
Boil the water. Then add the sugar, one teaspoon at a time, to the water, stirring as you go. Keep adding until the sugar will not dissolve anymore. At this point, you can also add food coloring.
Remove the pencil and add the solution to the jar, being careful not to get any of the unsaturated sugar in. Then replace the pencil and let the string dangle in the liquid.
Leave the jar undisturbed for at least a day. After they have reached the size you want or have stopped growing, you can take them out and allow to dry.
Science Behind It
A supersaturated solution, the saturated solution that has been heated up and then cooled, is unstable because it contains more solute-the sugar, than dilute-the water. This causes it to be unable to stay in liquid form, so the sugar will form a precipitate as it comes out of the sugar. Over time, the water will also evaporate, causing it to become even more saturated and leaving the sugar crystals on the string.
Tips
This would be a good project to do over the course of a few meetings or in partnership with a cooking project.