In ancient times ships that were propelled by slaves were kept in time by the beating of a gong.
On one such ship they found that, with the gong at the back, the rowers at the front were always out of time.
The captain decided to move the gong to the middle of the ship so that all the slaves could hear it.
It was a heavy piece of equipment and the sailors on the ship couldn't lift it or even drag it.
After thinking for a while they decided that they needed some rollers to move it easily. The only resource they had were the slaves, as they had many spare slaves for when they were in battle and slaves were injured.
They managed, with spare oars, to lever it up onto the body of the first slave then slowly move the gong to the middle of the ship.
As they moved it they sang.
"We're rolling a gong on the chest of a slave".
I think I know where my coat is.