Implants are made of a durable silicone shell that can withstand tons and tons of pressure. Simply squeezing them with your hand will probably not rupture the implant - you'll do more harm to your breast tissues than the implant itself. It would take a lot of force to rupture them, such as a high speed accident or fall from great height. However, ruptures may also be caused if punctured or if their shell gets more fragile (usually with age).
A breast implant rupture can be caused by surgical mistakes, physical trauma, general wear and tear, or capsular contracture. Removal of breast implants is necessary following rupture.