PUZZLEBOXX

When Derek handed the matchboxes to Danny, Danny secretly removed the inner tray from one of them.  When he placed the three matchboxes on the table he made sure that this "empty" one was in the middle.  Then, as he adjusted their position on the table, he carefully pushed the trays of the outside boxes into the centre (empty) box.  Of course, now that they are all joined together, Danny can do all kinds of "tricks" with them.