What's in a kilogram?
A precious cylinder of metal kept under lock and key in a temperature-controlled room holds the key to that answer.
But over the years, the international prototype in Sèvres, France - against which all other weights are measured - has been losing mass slowly, making the gold standard for the weight a less accurate measure of what one kilogram is.
Metrologists (measurement scientists) started to notice something amiss more than two decades ago.
Replicas all over the world were sent back to the International Bureau of Weights and Measures in Paris - the global custodian for weights and measures - for verification, and they were found to weigh more than what they were supposed to, showing that the original had probably lost weight.
How units are defined
The metre was defined originally in the 1790s as one ten-millionth of the distance from the Equator to the North Pole.
A prototype metal bar was later used as a reference for the metre. In 1983, metre was defined as the distance travelled by light in a vacuum in a specific fraction (1/299,792,458) of a second.
The second was defined originally as 1/86,400 of a mean solar day, as determined by the rotation of Earth.
However, astronomical observations during the 19th and 20th centuries revealed that the mean solar day is slowly lengthening due to the tidal force of the Moon.
The second is now defined based on the time it takes a Caesium-133 atom to oscillate 9,192,631,770 times at a temperature of 0 Kelvin (-273.15 deg C).
The kilogram is the only measurement unit that is still defined by a physical object.
That is likely to change to one based on a formula using a stable fundamental constant in 2018 when member nations, including Singapore, meet in Paris for the General Conference on Weights and Measures.
At the start, a kilogram was measured based on the mass of a cubic decimetre (one litre) of water at 4 deg C. Water is at its densest at this temperature.
Currently, the definition of kilogram is pegged to the mass of the International Prototype of the Kilogram, an artefact made of platinum-iridium in Sevres, France.
The difference might not seem like much - 50 micrograms, or smaller than the mass of a speck of dust - but scientists knew it would have implications for accuracy that might be amplified further down the road.
Moreover, other measurements, such as the ampere (the measurement of electrical current), are measured in relation to the kilogram.
What it's all about
Today is World Metrology Day - an annual celebration of the Metre Convention which was signed in 1875.
That year, representatives from 17 countries, including Brazil and the US, gathered in Paris to sign the treaty.
This meant that, for the first time, there was an agreement to standardise measurement units across countries.
Physical units such as the ampere were looked at after the Metre Convention was signed.
Singapore signed the treaty in 1994.
The theme of this year's World Metrology Day, Measurements in a Dynamic World, was chosen to align with how rapidly measurement science is changing.
To mark the occasion, the National Metrology Centre will be hosting a conference today. Speakers will talk about how measurement sciences and their applications can help industry.
"They plotted a chart... and found that the mass of these prototypes was mostly skewed towards one direction in comparison with the international prototype of the kilogram," said Mr Lee Shih Mean, a senior metrologist at the Agency for Science, Technology and Research's National Metrology Centre.
No one really knows why the international prototype of the kilogram is losing mass, but metrologists are trying to find the most accurate formula that would help define "1kg". For decades, they have strived to retire "Le Grand K" - as the platinum-and-iridium cylinder is called. It is the only SI unit left still based on a physical object.
Now, at last, it looks as if they have the data needed to replace it, with a definition using "fundamental constants in a standardised formula": by counting the number of atoms in a silicon sphere, or by equating the electrical and mechanical power in an apparatus.
Scientists are coming close to refining the definition, and member states will meet in 2018 at the General Conference on Weights and Measures in Paris to make their final decision. If all goes according to plan, the 1kg artefact will become redundant.
The earliest measurements of mass and metres were all based on physical objects. The metre, for instance, was originally defined in the 1790s using the distance from the equator to the North Pole (through Paris) as a reference. As for the kilogram, it was defined by measuring the mass of one litre of water when it is at its densest at about 4 deg C.
World Metrology Day today marks the Metre Convention signed by 17 countries in 1875.
This treaty was significant because, for the first time, nations around the world were able to decide on a common standard in the midst of growing international trade of manufactured products. Trade had been chaotic before then as there was no way to tell if someone was getting his end of the bargain, said National Metrology Centre executive director Thomas Liew.
An accurate scale was a way to expose cheats in trading and helped show the equivalence of two things one otherwise cannot exchange, which is critical in many economic transactions, added Dr Liew.
Today, drug companies need precise measurements for dosages, while amperes are required to ensure that appliances remain within safe limits.
Even mobile phones are the result of miniaturisation made possible only because of precise measurements for tiny computer parts measured right down to the nanometre, or billionth of a metre, explained Dr Liew.
"Measurement science has changed the world. Our ancestors didn't have a clock. They knew day and night but they didn't know how to partition their time to organise their day effectively. Today, it's no longer a problem.
"Accurate measures are important for trade, science, industry and our quality of life."
Correction note: An earlier version of the story stated that National Metrology Day today marks the Metre Convention signed by 17 countries in 1875. This is incorrect. It should be World Metrology Day. We are sorry for the error.