I was looking for a reading to give as a homework assignment during our Ecology and Taxonomy Unit. I could not find what I was looking for, so I wrote this. I decided on five characteristics of all living things. I did not include being made up of cells, or having DNA. Both could be added to your discussion. There is a pre-reading question and then four questions for after the reading. I have included an answer sheet. If you are looking for an activity for a substitute teacher, this could easily be done as an in-class activity covering one class period.
After investigating the diversity of life through the Taxonomy Sorting Activity, and Taxonomy Photo Activity I have listed, I asked the students to write down some of the characteristics shared by all of the different organisms we looked at. This activity stands alone, but is also a good activity at the beginning of a discussion of Ecosystems. Ecosystems include both living and non-living elements, and this will give you a working definition of what is alive.
As part of your discussion of what is living and what is not, consider making a list of things, both living and not, and asking students to discuss and decide if they are alive. There is an interesting Popular Science article summarizing a 2013 study of the bacteria found in clouds. Are clouds living? How about oranges? Steak? Acorns?