How hard was it to build Stonehenge?

The giant stones of Stonehenge are believed to have been dragged hundreds of miles across the countryside when it was built in prehistoric times.

But how was it done?

Students at University College London have been attempting to answer the question - with an experiment in the centre of London.

They attempted to transport a one tonne load using a wooden sledge - and lots of muscle.

PhD student Barney Harris explains.