A warning for Aboriginal and Torres Strait Islander listeners - this programme contains the names and voices of people who have died.

Max Pearson presents a collection of the week's Witness History episodes.

We take a look at the Ice Bucket Challenge, the viral fundraising sensation that took over the internet in 2014.

Our guest Professor Sander van der Linden breaks down the psychology behind virality and outlines the challenges facing those who conquered the algorithm.

Plus, how one man smuggled punk rock over the Berlin Wall.

Also, we meet the man who found a retirement home for Bulgaria's dancing bears.

We hear the remarkable story of Australia's Freedom Riders who campaigned against indigenous discrimination.

Finally, we relive the mountain top escape of the Yazidi's who were fleeing Islamic State Militants.

Contributors: Nancy Frates – Pete Frates mother. Sander van der Linden - Professor of Social Psychology at Cambridge University. Mark Reeder - smuggled punk rock over the Berlin Wall. Dr Amir Khalil – founded the sanctuary for dancing bears. Darce Cassidy and Gary Williams – involved in the Freedom Rides. Mirza Dinnayi - helped evacuate the Yazidi's.

(Photo: Ice Bucket Challenge. Credit:Getty Images)