Changing the Border
This is a nifty puzzle whose solution is based on the area of a triangle. The land is divided into two parts (P and Q) by the border ABC. Redraw a straight border line passing through point A so that each of the areas does not change.
http://www.ies.co.jp/math/java/geo/tochia/tochia.html

Dividing a Quadrilateral
This is a more challenging problem that is based on the same underlying concept.
http://www.ies.co.jp/math/java/geo/shi2to/shi2to.html

Help the missionaries and cannibals cross a river. The rowboat holds a maximum of two persons at a time. Beware: If the cannibals outnumber the missionaries on either bank, the missionaries will be eaten up.
http://www.learn4good.com/games/puzzle/swf/logic2.swf

Send all of the yellow frogs to the right side of the pond, and all of the red frogs to the left side of the pond. How many moves does it take?

http://www.hellam.net/maths2000/frogs.html

Sniff out the clues and find the sneaky thief. A little bit of deductive reasoning, and an whole lot of fun!

http://pbskids.org/fetch/games/csi_1/game.html

When the centres of any three coins lie in the corners of an equilateral triangle of some size, such coins form an equilateral coin triangle. How many equilateral coin triangles of different sizes can you count in the figure?

The object of the puzzle now is to remove the minimum number of coins so that no equilateral coin triangles remain. In other words, centres of any three coins among those that remained don't lie in the corners of an equilateral triangle.

http://www.puzzles.com/PuzzlePlayground/CoinsAndTriangles/CoinsAndTriang...

Students use the concept of Venn Diagrams to place each animal in the correct enclosure.
http://pbskids.org/cyberchase/games/logic/

Two mind reading activities. The first one can be explained using knowledge of linear expressions, while the second one is just a bit of fun.

A triangle is made up of four pieces. Move the pieces around to make a new congruent triangle. Except the new triangle is missing one square unit! Where did it go?

Your calculator is missing some of its keys! Which answers can you still get?
Six levels, from easy to challenging!

Alphametics are arithmetic puzzles in which each letter represents a different digit. Use your knowledge of place value and numbers to solve the puzzle.