What makes the brain complex isn't the number of neurons — it's the connections. Each neuron receives ~10,000 inputs AND sends ~10,000 outputs. No other system we know of has this density of interactions.

A synapse is where one neuron passes a message to the next. In a chemical synapse:

1. Neuron fires → an action potential (electrical spike — like a spark) travels to the tip
2. Tiny vesicles burst open, releasing neurotransmitters into the gap (chemical messengers — like throwing keys across a crack)
3. Each neurotransmitter fits its receptor like a key in a lock → the receiving neuron's voltage goes up (closer to firing) or down (pushed away from firing)

The whole handoff takes ~1–2 ms. Rare electrical synapses skip the chemistry — two neurons physically touch and current flows instantly, but the signal can't be shaped or filtered.

Synaptic plasticity: the more you repeat something, the stronger those specific neural connections become — and the weaker the unused ones get.

Think of learning a language, a skill, or a habit: every repetition is your brain literally rewiring itself. The connections you fire together grow together. This is not a metaphor — it is the physical mechanism of memory and learning.