Think back to a really vivid memory.

Got it?

Okay, now try to remember what you had for lunch three weeks ago.

That second memory probably isn't as strong,

but why not?

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 in 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.

Neurons throughout the brain communicate at dedicated sites

called synapses

using specialized neurotransmitters.

If two neurons communicate repeatedly, a remarkable thing happens:

the efficiency of communication between them increases.

This process, called long term potentiation,

is considered to be a mechanism by which memories are stored long-term,

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 5% of its neurons every decade

for a total loss of 20% 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.

Mental and physical health problems, which tend to increase as we age,

interfere with our ability to pay attention,