Theory:

What is conversion?
Conversion is the process of changing something from one state to another. Many people are probably familiar with this term related to religious conversion or two-point conversions in football. In math, we often convert one unit to another unit. That is, changing rupees to paise, metre to kilometre, litre to cubic metre, etc.
Conversion of solid
Conversions or Transformations becomes a standard part of our day to day life. Let us see the conversion of solid from one state to another through a real-life example.
Example:
We are sure everyone in our childhood played with clay. We transform clay into different shapes of animals, birds, toys and, etc (assuming there is no wastage in conversion). The volume of the new shape clay will be the same as the volume of the earlier clay. When we convert a solid totally from one shape to another, the volume of the shape is preserved, but the surface area changes.
 
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A video showing some more real-life examples of conversion of a solid shape.