To check a model rocket for stability one of the easiest things to do is a swing test. To do this you put a motor in the rocket, tie a string at the point where it balances level, and swing it around (the string should be 10' or so long for this). If the rocket flies straight, it's stable. Or so the theory goes.
But I don't always have motors available for this so I did some research and some math and determined that an Estes D sized motor is almost the same weight as 18 pennies, as long as those pennies were minted after 1982 (pennies were heavier before then). An E sized motor weighs the same as 22 pennies, approximately.
So I put 18 pennies in my rocket and proceeded to fling them all over the yard. No, not really. I did tape them together and shove them in for the first few tests, however, and then I thought there has to be a better way!
That's how I came up with these motor plugs. The D sized plug is sized like a D motor and has a void inside that holds 18 pennies centered within it. The E sized plug, in a similar fashion, holds 22 pennies centered within it.
Adding the weight of the plastic they probably come in slightly higher than the motors they represent but for the purpose of checking stability, having a little too much weight in the nose is better than not having enough, so this is ok.
The cap of each plug is stamped with the size of the motor they represent and the number of pennies that should be inserted.
I've added a 13mm motor plug because I needed to balance a rocket that took 13mm motors and didn't have one. Obviously you can't put pennies inside a 13mm tube (unless you have some way of shredding them) so this one is weighted with BB's. By my best guess, 24 BB's is roughly the weight of an A10-3T, the biggest 13mm motor. So voila! A 13mm motor plug that is the same size and (when filled with 24 BB's) weight as a 13mm motor.
I've not done an 18mm one yet. I've not needed one. But I am sure I will at some point.