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Why are glucose, fructose, and galactose called isomers?

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Tall, dense stalks of sugarcane grow on both sides of a narrow path, with green leaves arching overhead and sunlight illuminating the plants. — sugar Lines of sugarcane before harvest.

Glucose, fructose, and galactose are classified as isomers because they share the same molecular formula, $C_{6} H_{12} O_{6}$ , but possess different structural arrangements of their atoms.1

Because of these distinct structural configurations, these sugars exhibit different physical and chemical characteristics. For example, the specific arrangement of hydroxyl groups within the molecule influences properties such as the sugar's degree of sweetness and its biological function. While they are composed of the same number and type of atoms, the way those atoms are bonded and spatially oriented makes each a unique chemical entity.12