Why do we remember some things and not others, and why do memories eventually fade? Let's look at how memories form in the first place. When you experience something like dialing a phone number, the experience is converted into a pulse of electrical energy that zips along a network of neurons. Information first lands and short term memory where it's available from anywhere from a few seconds to a couple of minutes. It's then transferred to long term memory through areas such as the hippocampus and finally to several storage regions across the brain. But how do some memories get lost? Age is one factor. As we get older, synapses begin to falter and weaken, affecting how easily we can retrieve memories. Scientists have several theories about what's behind this deterioration. From actual brain shrinkage, the hippocampus loses five percent of its neurons every decade for a total loss of 20 percent by the time you're 80 years old, to the drop in the production of neurotransmitters like acetylcholine, which is vital to learning and memory. These changes seem to affect how people retrieve stored information. Age also affects our memory making abilities. Memories are encoded most strongly when we're paying attention, when we're deeply engaged, and when information is meaningful to us.