system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 Where does the electricity in our body come from?
gpt
Electricity is basically the movement of charged particles. A voltage is an excess of positive (or negative) charges on one side of a barrier. An electric current is the movement of charged particles in a unified direction (for instance, through a copper wire). A voltage can induce an electric current just like a difference in water pressure can induce a current of water (e.g. water will flow from a high to a low container if you link those containers with a pipe). The voltage does this because like charges repel each other: positive repels positive and negative repels negative. So if you have a lot of positive charges in one place, those positive charges will try to push each other away, and if you give them an "opening", positive charges will get pushed out (and the same goes for negative charges if there are a lot of negative charges in one place). 

The electricity we use in our homes is based on the movement of *electrons,* which are small negatively charged particles. The electricity in your body is based on the movement of ions. Ions are atoms that have lost or gained an electron. For instance, if you dissolve sodium chloride (table salt) in water, the sodium chloride molecules will split into atoms, with the sodium atom "donating" one electron to the chloride atom (we won't go into why they do this - suffice it to say that some atoms like to lose or gain electrons if they can). This results in a sodium ion (Na+) and a chloride ion (Cl-). The sodium ion is positively charged, and the chloride ion is negative.

Your body uses ions to create electric voltages and currents. For instance, nerve cells use sodium, chloride and potassium ions. Each nerve cell has "pumps" in its cell membrane that can move ions from inside to outside the cell, or vice versa. This process, which takes energy, causes more positive charges to build up on the outside of the cell membrane, and more negative charges on the inside. Thus, a voltage is created. Then, if the cell opens gates in its membrane that ions can pass through, an electric current happens: negative ions flow out of the cell and positive ions flow in. These pumps and gates are at the root of how the body creates and uses electricity in nerve cells. The whole story is more complicated than that (and beyond the scope of this answer) but those are the main principles.