Why can't monosaccharides be hydrolyzed?

Monosaccharides or the simplest form of sugar can't be hydrolyzed.

Because they can't get broken into another pieces; as they don't possess any achiral carbon atom as like in disachharides & polysachharides. Also they exist in their furyl or pyranose form leading to greater stability and harder to break . Glucose in pyranose form

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