Answer:
Radioactivity is the phenomenon exhibited by certain elements of spontaneously emitting radiation resulting from changes in the nuclei of atoms of the elements.
Two examples are:
Smoke detectors, because most of the residential smoke detectors contain a low-activity americium-241 source.
And watches, modern watches sometimes use a small quantity of hydrogen-3 (tritium) or promethium-147 as a source of light.
