All theoretical of course, I haven't tried this 
An aeroplane is on a MASSIVE runway sized treadmill. As the aeroplane pulls away to take off down the runway/treadmill the treadmill in turn reverses at the same speed. As the plane reaches 50mph going forwards, the treadmill matches the speed of the plane by going 50mph backwards.
Question...
Will the plane ever take off?
An aeroplane is on a MASSIVE runway sized treadmill. As the aeroplane pulls away to take off down the runway/treadmill the treadmill in turn reverses at the same speed. As the plane reaches 50mph going forwards, the treadmill matches the speed of the plane by going 50mph backwards.
Question...
Will the plane ever take off?