My take on Ken G's post (well, one take; there are a few): you could count the exact number of electrons, protons, and neutrons in each (assume each weighs exactly 10 kg). You could consult a table giving the rest mass of each particle, to 6 or even 10 significant digits. With some multiplication and addition, you would get three numbers, one each for the brick, bucket of water, and iron bar. The numbers will not be the same, even though they weigh the same - do you know why?

Yet - and this is truly marvellous - all three 'fall the same'!