Anyone seen that youtube video with the scottish or irish dude not understanding that 1 kilo of feathers weighs the same as 1 kilo of iron, even though iron weighs more than feathers?

Riddle me this /sci/. If mass and energy is equivalent, and iron has a different heat capacity then feathers, doesnt 1kg of iron at room temperature have more mass than 1kg of feathers? What if the speed of iron and feathers is different?