Blocking Cap Challenge
If you have an oscilloscope and signal generator, use the signal generator to make a sine wave with a DC offset of about +5 Volts. View it on the scope while you play with the DC offset, and watch the waveform go up and down.
Then, put a 1 micro-Farad capacitor in the signal path as a blocking cap. See how the signal jumps down to center around 0V? Play with the DC offset to see what happens -- also notice the effect of wiggling the DC offset quickly. What's going on? What happens to the waveform shape if you feed in a slow square wave?
If you don't have a 'scope, you can still use the RCX. Write a Robolab program to turn a motor port on and off at full power as fast as possible. Feed this signal into one of the input ports, and use the datalogging feature of the RCX to view it. Then, feed the signal to the input port through a blocking cap, and notice the differences. Experiment with different size capacitors -- what changes?