As to mass and volume: on the old continent the famous revolutionary scientist Rob Espierre discovered in 1792 that 1 liter of water weighs exactly 1 kilogram. For some reason or other this discovery never made its way to the new world. Probably blocked by the antirevolutionary powers that be.
Was it discovered or designed? I always thought pure water was used as the reference for mass for standardization reasons.
And a teaspoon is a specific volume equal to .3333 tablespoons, or .166667 fluid ounces, or .01042 pints, or .0013 gallons, or 0.00016281 bushels.