The plural of solid; substances that maintain a fixed shape and volume, or in infant nutrition, solid foods as opposed to liquids.
From Latin 'solidus' meaning firm or whole. The physical science meaning developed in the 17th century, while the infant feeding context emerged in the 20th century with modern pediatric practices.
In physics, the distinction between solids, liquids, and gases seemed clear until scientists discovered states like plasma, Bose-Einstein condensates, and materials that behave as both solid and liquid simultaneously. Baby 'solids' are often actually quite mushy, showing how context shapes our use of scientific terms in everyday life.
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