Deniz Sarikaya is a guest researcher at the University of Copenhagen and the Technical University of Denmark (funded by the DAAD) and postdoctoral researcher at the Vrije Universiteit Brussel (within an FWO-project). Lukas Baumanns did his doctorate on problem posing at the University of Cologne. Currently he works at the Chair of Special Education in Mathematics and focuses on early mathematics learning and mathematical difficulties. Karl Heuer is an assistant professor in Discrete Mathematics at the Technical University of Denmark. He mainly works in Graph Theory, but is also active in enrichment programmes. He obtained his Ph D in mathematics at the University of Hamburg. Benjamin Rott is a professor of Mathematics Education at the University of Cologne. He obtained a Ph D at the University of Hannover, worked at the University of Education Freiburg and the University of Duisburg-Essen. His research interests include problem posing/solving, giftedness, and beliefs.