The stage of tertiary circular reactions is when infants begin experimenting with thought and deed.

The stage of tertiary circular reactions, according to Piaget's theory of cognitive development, is when infants between 12 and 18 months old begin experimenting with thought and deed by intentionally repeating actions with variations to see different results. This includes actions such as dropping a toy to see if it makes noise, or shaking a rattle to hear different sounds.