See instructions below

Instructions
Just move the blocks around using your mouse.
You can select a block by clicking on it.
You can select more than one block by shift-clicking on other blocks (click on them while holding the shift key down)
You can also select blocks by drawing a rectangle on the screen surrounding them.
Once blocks are selected you can:
Join them (when joined they automatically cut themselves into 10s), or
Break them into "1" blocks
You can add new blocks by dragging them from the "New Blocks" box.
You can delete blocks by selecting them then pressing "Remove".

Uses
Demonstrate addition facts (example: 4+1=5), subtraction (example 5-1=4), multiplication (example 4 2s make 8), division (6 can be broken into 3 2s, or 2 3s) ... and more!
Make nice patterns: pyramids, squares, rectangles ... even shapes like cats.