The regular bulb - on top - has a thin wire filament that heats up and produces the light. The bimettalic strip in the flasher is setup so that when the light is off the contact and the bimetallic strip are touching. This causes the filament to glow and produce light. The green metal expands at a faster rate than the blue when the strip is heated, so the bimetallic strip bends down. This opens the gap with the contact and the light goes off. The metal quickly cools and makes contact again. In this way there is a steady flashing of the bulb.

Flashing Bulb Diagram

The white junction beween the two metals can be seen in this colorized SEM image.

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How mini-lights work

Now - the heavy stuff - How blinking lights work.