Tap <code>D7</code> then <code>digitalWrite</code> in the popup. Now when you tap the <code>D7</code> circle the tiny blue LED should turn off or on! This is because the LED shares a connection to the Core with the pin labeled D7.
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You could hook your own LED up to the Core on another pin and do the same thing, use digitalRead to tell that a switch has been pressed, or analogRead to see the position of a knob.
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You can always get Tinker back on the Core by following [http://docs.spark.io/#/tinker#tinkering-with-tinker-the-tinker-firmware these instructions].