Ions are atoms or molecules with an electrical charge, positive or negative. Their motion is governed by a simple rule: unlike charges attract, like charges repel. These forces come from electrons, which determine how strongly ions pull on or push away from each other. Inside cells, there is an electrical potential difference across the membrane—like the two ends of a battery. This potential drives ions through channels, as if they were sliding downhill along an invisible slope. Within the channels themselves, charged amino acids add their own influence. Their electron clouds form fields that guide specific ions, pulling them in or blocking their path. The same principle also operates outside biology. In ion thrusters used on spacecraft, electric fields accelerate ions until they shoot out as a fast jet, pushing the craft forward in the vacuum of space. Whether inside a nerve cell or on a journey to Mars, the movement of ions always follows the same invisible law of electrostatic force.