Spontaneity is one of my favorite aspects of teaching. It happens in the classroom when I think of a story that I can tell my students because it is related to whatever we are learning, or when something unusual happens at school, or when a student has a great idea or does something appropriately funny (like tries to taste our salt and flour volcanoes during art class.)
Teacher-led, or teacher-controlled spontaneity gives students a sense of wonder and excitement, and makes school seem much more alive and fun.
It keeps class upbeat and flexible. A spontaneous happening shouldn't usually last more than a minute, and is sometimes just a few seconds long, but it keeps the students paying attention, more focused, more enthused, and always wondering what might happen next.
There are two types of spontaneity: genuine and planned. If enough real spontaneous stuff is happening, you don't need the planned. If it isn't, you can create some whenever it is needed. (It is usually needed a lot in January and February.) It always needs to be controlled; the students need to know the rules and what you will and won't allow, and after the spontaneous event, students should know that it's back to business.
For examples and stories, click on the links below.
Spontaneity Part I: Genuine Spontaneity
Spontaneity Part II: Planned Spontaneity