I'll expand on Phil's answer. When an object has a Normal and a Selected state, it will automatically change to Selected when it is first clicked. It will change back to Normal if the user clicks it again. In other words, it toggles between Normal and Selected, without the need for any triggers.
Note: That behavior is a bit different if the object is part of a button set. When objects are in a button set, only one of them can be selected at a time. In other words, they function like the responses in a multiple-choice question. (Details here: https://community.articulate.com/series/articulate-storyline-360/articles/articulate-storyline-360-user-guide-how-to-work-with-button-sets )