I love using rectangles as a model for multiplication. In this video, Mike & son offer a pithy demonstration of WHY a negative number times a negative number has to come out positive:
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Denise Gaskins' Let's Play Math 
o.m.g. They’re fabulous! I’m going to have my college students watch this.
James Tanton has a video that does the same thing on a whiteboard, just drawing the rectangles, but I think it’s even more magical to see it acted out with the blocks.
That’s really neat! Really shows why in this case minus X minus has to be a plus.
I must use manipulatives to represent the grid method for multiplication in class!
Nicely done!