Hi, I thought you may like this Mentalrobics exercise from Braingle.com:
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Transience

Transience refers to the fact that memories fade with time. Different memories fade at different rates for different people. This is mostly due to the frequency with which the memories are remembered.

Each time you recall a memory the neural pathways [ For more on 'neural pathways' go here: http://www.braingle.com/mind/317.html ] that make up that memory get triggered. This causes these neurons to strengthen and ensures that this memory will stick around a bit longer. If you rarely recall a memory, it will naturally fade into the background because it is probably unimportant.

This fact is why repetition and review are such important steps to do if you want to remember something. This is especially important for new memories, which are very fragile and can fade quickly unless you strengthen them.
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For more mind exercises: http://www.braingle.com/mind/