Why does a balloon stick to a wall after you rub it on your hair?

Rub a balloon against your hair and hold it up to a wall, and it will stick there, seemingly defying gravity, with no glue and no trick involved. The explanation starts with the atoms that make up both your hair and the balloon: ordinarily, atoms are electrically neutral, carrying equal numbers of positive protons and negative electrons. Static electricity, an imbalance of electric charge sitting on a material's surface, can only appear once those positive and negative charges get separated from one another.

Rubbing does exactly that. Whenever two surfaces touch, or slide against each other, and then pull apart, electrons can hop from one material to the other, leaving one surface with extra positive charge and the other with extra negative charge, a process called the triboelectric effect. It's the same mechanism behind the classic science-class demonstration of rubbing fur against an acrylic rod, and it's the main reason static electricity shows up in everyday life at all. The word 'static' exists specifically to separate this stuck-in-place imbalance from ordinary current electricity, where charge actively flows through a conductor like a wire.

Rubbing a balloon on your hair leaves the balloon negatively charged, loaded with extra electrons it picked up from your hair, which is exactly why your hair often stands up afterward too. Bring that negatively charged balloon near a wall, and it doesn't need to touch anything else to feel an effect: the wall's own positive charges get pulled toward the balloon while its negative charges are pushed away, and because closer charges pull harder than distant ones, the balloon and wall end up attracted to each other strongly enough that the balloon clings there, suspended against gravity.

Static electricity isn't only caused by rubbing, either. Simply squeezing or stressing certain materials can generate a separation of charge, and so can heating them. And when a stored imbalance finally gets neutralized, for example as your finger nears a conductor, you feel, hear or even see it as the small spark of a static shock.