Why does paper curl when burnt?

Have you ever seen paper curling when it's burnt?
No? It isn't too late. Just try burning a sheet of paper. It curls like this.

But why? 

See, the basic constituents of paper are cellulose fibres and additives like china clay and chalk. Common paper which we use for daily chores, contains around 5% water when dry(believe it or not!) While burning, it loses water and eventually it becomes ash.

Since paper is not a good conductor of heat, heat is not transferred to the entire lamina. When we burn one of the corners, only the region in the vicinity gets heated up and loses water due to dehydration.

The cellulose fibre, being an organic polymer, tends to stretch when it gets heated up while clay and chalk tend to take up(absorb) heat without undergoing any change(state).

As the sheet of paper is being burnt, the heated side loses water faster than the other side. Hot air always rise upwards as it is less denser. Analogous to this, the heated side being less denser tends to rise upwards. But the trace amounts of clay and chalk play a significant role in absorbing the heat supplied. So, the other side tends to remain as it is. Hence it curls upwards.

Now, our part is in analysing the curl of different sheet of paper differing in thickness. It is obvious that thicker paper curls more. This is because, in thick sheets, the two sides are farther apart than in thin sheets.

Also, paper made of different materials will curl to different extent. This can be explained by the percentage composition of water in different types of papers. Percentage of water in dry sheets of paper varies from 2 % - 10%. Newspapers have the maximum percentage of water(around 10%). Even-though we have really "hot-news" on newsprint, it is the coolest paper of all!

Try these for better understanding:

#1. Take a plain sheet of paper and hold it such that the plane is horizontal. Now, burn one of the corners. You will see it curls upwards. Now just turn the sheet upside down(180 degree, it's still horizontal). Even now, it curls upwards. 

#2. Take a thick sheet of paper(post card-thick preferred) and gently wet one of the sides with water. Now, burn it. You can see only the dry part curls.

#3. Roll a sheet of paper and burn it. Curling is seen in that direction which is concave up.