bummee, bunn, up-pee, and bike

Max is picking up new words at an incredible rate.  Some examples:

• up-pee.  Means, "up, please."  Said while grappling an adult's calf with head bent way back to look up at his audience, sometimes in a whiny tone.  It took us a while to get him to say "up" when he wants to be picked up.  But the "please" part came out of nowhere -- a pleasant surprise.
• bike.  Means, "bike."  Max loves to play with bikes.  If someone's riding, he pushes along.  If the bike's lying on its side, he'll spin the wheels.  In all cases he repeats "bike" with great enthusiasm.
• bummee.  Means, "bummer!"  He learned this one today after just one repetition.  He overheard me say, "bummer" to myself and thought it was hilarious.  He laughs out loud and repeats several times whenever I say it.
• bunn.  Means, "button."  Max does not distinguish between buttons sewed onto clothing and belly buttons.  He loves belly buttons, though.  He likes to lift up people's shirts and jam his finger into their belly buttons.

Max is now at that stage of development that drives neuroscientists crazy.  How do kids this age pick up language so quickly?  Why can't adults do it that quickly?  For that matter, how do our brains allow us to learn any language at all?