When the mug is cold, you'll see how your brain looks before the infusion of caffeine. As you fill it up and the mug warms, the design changes to show your wide-awake, post-caffeine brain.
Here's the short story on the science of what's happening:
As adenosine naturally builds up in the brain, it binds to the A1 receptor and keeps dopamine locked up. Caffeine works its magic by stealing adenosine's spot on the A1 receptor, which releases dopamine into the synaptic gap. This rush of dopamine imparts the happy, alert feeling you love about your magical brew.