Cells make tissues.
Tissues make organs.
Organs make organ systems.
Organ systems make organisms.
Get your students up and moving, while helping them understand the organization of organisms.
Prepare the following 2 of each of the following signs (Notice I didn't do much to prepare these - printed them out and stapled them to a piece of construction paper, just for a little added weight - this was a last-minute idea that was quickly assembled after lunch one day).
Blood Cell I
Blood Cell II
Heart Cell I
Heart Cell II
Brain Cell I
Brain Cell II
Nerve Cell I
Nerve Cell II
Stomach Cell I
Stomach Cell II
Intestine Cell I
Intestine Cell II
Hand each student a sign. They are each a cell, wandering around the classroom on their own.
Cells make tissues, so have them find their matching cell, in order to make a tissue. You now have pairs of students walking around together.
Tissues make organs. Have all the heart tissues combine, all the stomach tissues combine etc. You now have organs, a.k.a. groups of four students.
Organs combine to make organ systems. The heart and blood make the circulatory system. The brain and nerves make the nervous system. The stomach and intestine make the digestive system.
And finally, organ systems combine to make an organism. At this point all the students (cells) come to gether to create one organism.
If you wanted to have a little more fun, you could turn it into a game of Simon Says and call out the different levels of organization (i.e. cell, tissue, organ, organ system, organism) and students would have to quickly assemble into the appropriate grouping.