How does hydrogenation of polyunsaturated fats make them more solid-like?

Organic molecules consist mainly of carbon, hydrogen, and (often) oxygen atoms. Saturated fats refer to those fats whose molecules contain all the hydrogen atoms they can. Unsaturated fats contain some double bonds between some of the carbons instead. Hydrogenation breaks the double bonds, making two single bonds for each carbon; one of those will stay between the carbons, and the others will attach to hydrogen atoms supplied from some other substance. A very simple example would be ethane: H3C-CH3, which is saturated, and ethene: H2C=CH2, which is unsaturated. By breaking one of the two carbon-carbon bonds and substituting hydrogen atoms, the ethene will convert to ethane.