A riddle resolved

I remember a very long time ago, when I was a secondary school student, my physics teacher , Mr. Lau posed my class ( and other classes he was teaching too, I believe) a riddle. It was posed during a class on Newton's three laws.

There is a motorcycle under the MRT. When the MRT moves forward, the motorcycle moves back.Why?

And just a few minutes ago, when I was reading up on my physics textbook the answer appeared infront of me so glaringly obvious, I nearly went blind that moment.

But the thing I don't get is, how in the world did that problem given so long ago embed itself so deeply in my mind?