Grignard reaction
Mechanical methods include crushing of the Mg pieces in situ, rapid stirring, or using ultrasound (sonication) of the suspension.  Iodine, methyl iodide, and 1,2-dibromoethane are commonly employed activating agents.  Chemists use 1,2-dibromoethane because its action can be monitored by the observation of bubbles of ethylene.  Also, the side-products are innocuous: