What is a constitution?

A chilly January morning in Delhi: Republic Day! Marching bands, colourful displays, the tricolour unfurled at Kartavya Path. This day marks the Indian Constitution coming into effect on 26 January 1950.

The original Constitution is kept safe in a glass case filled with helium in Parliament. Why helium? It's a gas that doesn't react with paper or ink, so the pages and writing stay safe for a long, long time.

Our top officials promise to uphold it. The President takes an oath to preserve, protect and defend it. The prime minister, ministers and judges promise to follow it.

So what is a constitution? Simply: a document that sets out a nation's basic principles and laws. It lays out:

• the three organs of government (legislature, executive, judiciary) and their jobs
• checks and balances between them, for fairness and accountability
• the rights and duties of citizens
• the nation's long-term goals and hopes

Why do we need one? Picture a state kabaddi final. A fight breaks out: was a player 'out'? The referee pulls out a small official rulebook. The captains read the rules with her and agree: yes, out. Match over, no quarrel. Now imagine no rulebook, or captains who refuse to look at it! Games have rules so that people don't fight afterwards. Our Constitution is a rulebook for the country.

Its rules decide what kind of government we have, how it's formed and how it works: how laws are made and carried out, who elects the executive, how the judiciary is set up, and what rights and duties each citizen has.

It also states the values India stands for: equality, justice for all, fraternity, pluralism (many ways of life side by side) and freedom. Those values are the base for its laws.

Its size: it began with 22 parts and 8 schedules; today it has 25 parts and 12 schedules. It's the world's largest written constitution!