Is this goblet purple or brown? Well, it depends.

The cup is dichroic, meaning that it changes colour under different lighting conditions , from clear purple when light is transmitted through it, to opaque brown when light reflects off it.

Vittorio Saggiomo and colleagues at Wageningen University & Research in the Netherlands created the cup from gold nanoparticles and a 3D-printable clear plastic.

To do this, they mixed nanoparticles with citrate – a derivative of the acid that makes lemons sour – …