Ask each person to write any digit on a sheet of paper, large and dark, so it can be seen from across the room (or prepare sheets for the class). Divide the class into pairs, threes or fours or fives, or larger, depending on the size of the numbers you want to practice. Each student becomes the digit she holds. Call out criteria for the numbers you’d like, for example, "Show the largest number you can with your digits." Allow a minute for the groups to organize themselves, and then give them the signal to stand or turn so the whole class can see. Ask someone from another group to read the number and decide if it meets the criteria. With the whole class you can look at all the numbers, asking who has the largest, and smallest, or asking the groups to line up in order from largest to smallest. Here are some suggestions for the questions:
Whole numbers: make the largest possible number; the smallest possible number; a number with the largest digit in the tens place (or hundreds place, etc.)
Decimal numbers: Give each group a token (a hackysack, or a chair or a book bag) to use as a decimal and ask the same kinds of questions as above-the largest number possible, the smallest possible number, a number with the smallest digit in the tenths place, a mixed number, etc.
Variation: If you are working with only one or two students, give them several digits and ask them to line up the digits on a ledge or table.
You are going to ask the class to sort themselves into groups of any size that meet the criteria you call out, for example, "groups in which half the people are wearing glasses." Start by saying that there will nearly always be a group that doesn’t fit the criteria you call out, and that figuring out that you can’t make a group is an important part of knowing math, and that you will be asking that group to name a fraction that describes their group. Then ask the class to get into groups or pairs in which half the people are wearing glasses.
In this example, when students have finished getting into groups where half the people are wearing glasses, you might find several pairs of people. In each pair, one student is wearing glasses, the other is not. You might find a group of four with two people wearing glasses, or even a group of six with three people wearing glasses. And you will likely have an individual or a group of people left over, either all with glasses, or all without. You might have a group of three who want to stick together, with one wearing glasses and the other two with no glasses.
Start with the group that doesn’t fit the criteria: What fraction of the people in this group is wearing glasses? Ask them to be your assistants in checking out the rest of the activity.
Check all the other groups: "What fraction of the people in this group are wearing glasses?" (Your student assistants can ask the question, and count to check that the group mix is correct.)