It's just water and sugar, cooked into a syrup. Quite often flavoring is added--lemon juice, steep a vanilla bean in it, etc.
2 1/2 cups sugar
Full 1 cup water
Combine ingredients in a nonreactive 2-quart heavy-bottomed saucepan over medium-high heat and bring to a boil. All the sugar crystals should completely dissolve. Remove from the heat and pour into a clean medium-size bowl. Let cool completely before using. If you are short on time, you can cool the syrup over an ice bath. Simple syrup can be stored in the refrigerator, indefinitely, if kept in an airtight container.
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