Mercury is a liquid metal which conducts electricity.

At one end of the tube are two metal contacts.

When the tube is horizontal there is no mercury between these contacts.

When the switch is tilted the mercury completes the circuit.

One application of the tilt switch is in film projectors, where if the film breaks, the tilt switches off the power to the projector motor.

Fit one to your bicycle as part of an antitheft alarm,