These are the smallest unit of interaction that are caused by a trigger – either by the user, system, another user, data recognition, etc.
Help users recognize how to trigger something and what is triggerable that will result in an interaction. A button is a trigger. A glowing button is a more obvious trigger. Sometimes triggers are invisible, but there can be clues to suggest the interaction (these must be thoughtfully designed).
Make sure triggers are consistent. A 3D button sets a filter on this page. A second does the same. The third leads you to another page? These are all 3D buttons of the same attributes so users will think they have the same effect.
Triggers can incorporate the data behind them. For instance, a badge notification tells the user something.
Always keep in mind affordance. A 3D button should be clickable and produce an effect.
If the user has to keep triggering, make the trigger more visible. For instance, the “Publish” button can be stuck to a “frozen” header so that no matter where the user is in their canvas, they can publish right then and there instead of scrolling up.
A label can be useful for a trigger if the info isn’t conveyed through the trigger. The “shake” gesture on an iPad is often non-intuitive. If it has an effect, you may need a textual label that says “Shake”.