Draw a large circle on the board. All the numbers you write inside the circle follow a certain rule; none of the numbers outside the circle follow the rule.
As you add numbers inside and out, students try to determine what the rule is.
The easiest example is evens in the circle, odds outside the circle. Other possibilities include factors of 12; multiples of 4; divisible by 3 or 5 or . . . ; primes and composites; two-digits in the circle, one- and three-digits outside; numbers less than 20, numbers greater than 20; regular quadrilaterals in, irregular quadrilaterals out; or try a trick question on the 10 kids: only numbers written with straight lines (1, 4, 7) in the circle, only numbers with curved lines (0,3, 6, 8) out; this means you can’t use 2, 5 or 9 because they have both straight and curved lines. This game can also be easily adapted to other subject areas.
• fractions (even vs. odd denominators; lowest terms vs. not in lowest terms
• numbers that end in ___
• starting with 1 (or even ½), numbers in the circle double
• numbers with ___ in the tens place
• numbers in the circle have a number in the tens place one more (or one less) than the number in the ones place (ie: 32, 98, 76, or 23, 45, 67; to make it more challenging try 1098, 5432)
• numbers in the circle have digits that add to a certain total (57, 183, 48)
• numbers in the circle increase (or decrease) a set amount each time
• fractions (less than ½ outside the circle, greater than ½ inside the circle; ½ and fractions equivalent to ½ can’t be placed in or out, a clue for students)
• large numbers with consecutive digits inside them in the circle (e.g.: 1034588 and 267814 inside)
from:
http://washmath.org/files/GamesAndActivities.pdf