Sensor are nests whose birds 'live' inside the browser. They can be used the same ways as ordinary nests. The sensor above is listening for keyboard events (keydown events) and the bird reports the key attribute of the event. Try a few keys on your keyboard and watch the sensor below.
Unlike nests, the backside of sensors has fields that can be edited to change what event and what attribute of the event the sensor tracks. All of the browser events should work. Here is a good site that documents all the available events. Click on the sensor below and edit the event name and event attribute. For example, try changing the event name to click and the attribute to which to listen to mouse click events and see which mouse buton was clicked.
You can also obtain a sensor that is restricted to events that happen to a ToonTalk widget. If you click on a widget and then on > to see the advanced options where you will see a Make a sensor nest button. Clicking it will produce a sensor for events that happen to that widget (e.g. the mouse click that was on the widget). Try it on the number below.
Sensors are great for making interactive apps in ToonTalk, including games. Here's a simple example of an element that rotates when clicked. Click the stop sign to stop it and then click the picture to see how it works.
ToonTalk events can also be sensed. The first one supported is 'widget added'. It is triggered if something is added to an element widget, a box, or the backside of any widget (including the top-level work area). The event has a 'widget' property that provides a backside of the widget added so it can be inspected, altered, or removed. Other properties are 'where' which has the value of either front or back depending upon whether the front or back of a widget was added. If the widget was dropped in a box then 'index' indicates the hole number the widget was added to.
Here is a toy example using a 'widget added' sensor: