Try and link the nodes so that they all have a different number of links, apart from the different coloured one. You can't link nodes that are vertically above each other (shown by the gray lines). – Click a node to select it, then click another node to link them together. – Click on the stage to deselect. – Delete links the same way you make them.
From @bharvey — http://scratch.mit.edu/forums/viewtopic.php?pid=1501468#p1501468 A mathematician is at a party at which there are five married couples (one of which includes the mathematician). The mathematician notices that people are shaking hands when they meet, but nobody shakes his/her own hand, and nobody shakes his/her spouse's hand. The mathematician asks every other person "with how many people did you shake hands" and gets a different answer from each person. With how many people did the mathematician's spouse shake hands?
