How did the angels prostrate to Adam (peace be upon him)? What kind of a prostration was it?

It is obvious that that prostration was not the kind where one puts his/her forehead on the ground. Prostration is the symbol of obedience to the Divine orders at the highest level. It is also the ultimate boundary of modesty. This whole universe has been given to the service of the humankind and the humankind has been chosen as the caliph (master) of the earth as a divine gift. The angels prostrating to Adam is the declaring of this Divine judgment to the angels.

That prostration is a Divine order. This is a proposal for worship for the angels aside from the declaration that the humankind is superior to the angels and the Jinns (creatures with free will, made from 'smokeless fire' by God). They are obligated to prostrate as God ordered them to. The prostration to Adam was made to God in reality, because it is fulfillment of a Divine order.

We, too, turn to the kiblah (direction of Mecca in worship) when we pray. If we are next to the Kaaba (is the holiest place in Islam- it is around Kaaba that ritual circumambulation is performed during Hajj), we turn by facing to it, then pray and prostrate. This prostration is seemingly to the Kaaba but in reality to God.