Your topic is watching TV and the point you are making in this paragraph is that violence negatively influences behavior.
When students are just beginning to write a topic sentence, it might be ok for them to write "watching too much TV is harmful." But, as they become better writers, if students just wrote "watching too much TV is harmful," it really doesn't provide the reader with enough information about the paragraph. What about watching too much TV is harmful? What else is the writer going to write about harmful TV watching?
Teaching students the TOPIC + MAIN POINT technique helps students make sure their topic sentence is specific and focused, which allows them to further expand on the topic in an effective way.
Some activity suggestions:
Two activities I have liked to do to help them understand topic sentences are using sentence stripes and writing topic sentences for already written paragraphs. You could easily write a paragraph with a good quality topic sentence, mix up the sentences, and have the students put them paragraph back in order identifying the topic sentence and other parts of the paragraph if you want. The other activity I like to do is to pre-write some paragraphs that are missing a topic sentence. The students should then write a topic sentence that goes with that paragraph. Then, have the students share their topic sentences and they will see that while not everyone's is the exact same, they are fairly similar.
If you would like to download the activities suggested above please CLICK HERE. I also have a whole group PowerPoint lesson available to you.