Card Games. I like to use a variety of 2-sided informational cards, where kids can play by themselves or with a partner. It's best if the game requires the students to provide some sort of verbal explanation of the concept in order to win, rather than simply requiring them to tell the answer. Even if the game could be played with just short answers or matches, I try to add a verbal explanation component if possible.
Here is an example. This is part of a 16-card game called "Which Is Greater," which not only helps students decide which of two fractions is greater, but also requires them to explain their thinking. Fractions are compared on the basis of like denominators (7/8 vs. 3/8), different denominators with like numerators (1/3 vs. 1/6 or 2/3 vs. 2/9), and fractions that are equal to 1/2 (1/2 = 4/8). Card backs provide correct answers as well as detailed explanations of various possible reasonings.
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Coloring sheets. My students seem to like worksheets where the answers are all color coded, so I use them frequently in my math centers. I especially like them because, to some extent, the kids can find and correct some of their own mistakes when their answers don't match the color key. Coloring sheets are also nice to check, because in some cases, I can just look at the colors.
Here is the answer key for a mixed number addition sheet. I like this one because kids solve problems such as 4 3/8 + 1 5/8, and they get 5 8/8. Then they must convert 5 8/8 to 6 wholes in order to figure out what color to make it.
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Puzzles that spell out a word or phrase. Students also like math center puzzles that have a key that lets them use their answers with a letter code key, so it spells out an answer to a riddle or a hidden message of some sort. Again, I like that students can catch some of their own mistakes when their answers don't match the key. You can find these in some workbooks, but I sometimes just make up my own.
Here is an example. This one focuses on finding factors. I put some very tempting distractors into the key, and that prevented my students from using the sentence to avoid working out the math. The answer key is pictured.
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Mazes. If you have a lot of information that needs sorted, mazes are a really fun option. Students must identify the spaces that fit the description and follow those spaces to travel to the finish. It works well for skills such as odd/even numbers, multiples, and divisibility rules.
Here is an example. Students must follow the multiples of a given number to get from the start to the finish.
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Crossword puzzles. I sometimes put crosswords of my vocabulary words in my math centers. But, when I want students to practice converting word form to standard form, or perform some sort of computation, I sometimes put out "crossnumber" puzzles at the math centers. They are just like a crossword puzzle, but they have numerical answers. Again, kids tend to catch some of their own errors, whenever the criss-crossed answers don't overlap.
In the puzzle pictured to the right, students must read the word form of the number and write the number into the puzzle in standard form.
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These are are just a few of the types of activities I use to make math centers fun for my students. The students don't realize how much they are learning! Hopefully this helps give you some ideas to get your math centers rolling!