Inductors and iron rods

If you put two inductors close to each other and run a current through one of them, a current is momentarily induced in the other one. If you connect the inductors with an iron rod, the induced current is greater, and if you connect them with a U-shaped piece of iron, it's greater still. Why?

Bitches don't know about electromagnetism.
watch : http://www.youtube.com/watch?v=q_v2NpgEpws and the 20th lecture.
and if you don't understand watch them all.