Expand one side of the cylindrical paper tube to get a what or what may be what, and its area is

Because, when a cylinder is cut along the height, when the circumference of the bottom of the cylinder is equal to the height of the cylinder, the unfolded figure is square;

When the circumference of the bottom surface of the cylinder is not equal to the height of the cylinder, the unfolded figure is rectangular;

When a cylinder is cut obliquely instead of along the height, the obtained figure is a parallelogram;

If you cut the curve along the height direction, you can get an irregular figure.

Therefore, the side of a cylinder may be rectangular, square, parallelogram or even irregular.

So, the answer is: square, rectangle, parallel four sides, irregular figure.