On the other side of the synapse, attached to the membrane of the dendrite, are these things called receptors. There are receptors for many neurotransmitters. Let's say the magenta-colored ones are serotonin receptors and the green ones are for dopamine. Notice how a serotonin molecule can easily fit into the serotonin receptor, but not into the dopamine receptors (or any other type of receptor for that matter). This is because serotonin recepters are designed specifically for serotonin molecules. When a serotonin molecule attaches to a receptor, which is called receptor binding, the receptor sends chemical information down the dendrite to the cell body of the neuron. The cell body then decides, based on the information from all its recepters put together, whether or not to fire an electrical impulse down its own axon. If a critical amount of receptor binding occurs then the axon will fire, causing the release of other neurotransmitters into other synapses. This is how your brain communicates, and something like this is happening in your brain at a normal pace all the time.

Research has shown that your mood is influenced in part by the amount of serotonin receptor binding. When you are happy, it is likely that you have more serotonin receptors activated. Positive events in your life (like falling in love, perhaps) cause greater serotonin release, increasing receptor binding. So does taking ecstasy.

After a little while the serotonin molecule will detach ("unbind") from the recepter and float back into the synapse. When this happens, the receptor stops sending chemical signals to the cell body, and it waits for another serotonin molecule to come along.

(Those yellow things on the membrane of the axon terminal are serotonin reuptake transporters. Don't worry about them just yet.)