  Modern soft X-ray observatories can yield unique insights into time domain
astrophysics, and a huge amount of information is stored - and largely
unexploited - in data archives. Like a treasure-hunt, the EXTraS project
harvested the hitherto unexplored temporal domain information buried in the
serendipitous data collected by the European Photon Imaging Camera instrument
onboard the XMM- Newton satellite in 20 years of observations. The result is a
vast catalogue, describing the temporal behaviour of hundreds of thousands of
X-ray sources. But the catalogue is just a starting point because it has to be,
in its turn, further analysed. During the project an education activity has
been defined and run in several workshops for high school students in Italy,
Germany and UK. The final goal is to engage the students, and in perspective
citizen scientists, to go through the whole validation process: they look into
the data and try to discover new sources, or to characterize already known
sources. This paper describes how the EXTraS science gateway is used to
accomplish these tasks and highlights the first discovery, a flaring X-ray
source in the globular cluster NGC 6540.
