Why can't matter be destroyed or created

Atoms can be combined with other atoms to form molecules. Matter is anything that has mass and takes up space. ... Matter can change form through physical and chemical changes, but through any of these changes, matter is conserved. The same amount of matter exists before and after the change—none is created or destroyed

water at room temperature is flowing through a pipe with pressure taps located 10 ft apart along one section of the pipe, which are connected to a mercury (s=13.6) manometer.