1. Introduction: Same Shape, Different Size
Hold up your hand. Now picture its shadow on the wall, big and dark. The shadow is bigger than your hand. But it is still hand-shaped. Five fingers, same gaps, same bend of the thumb.
That is the whole idea of this chapter, sitting right there on the wall.
Think of two more things. A small photo on your phone, and the same photo printed big and pasted on a school notice board. The big one is wider and taller. But nobody is stretched. Nothing looks squashed. The face is the same face.
Stop scrolling. Try it in your head before reading on: what stayed the same between the small photo and the big one, and what changed?
The shape stayed the same. The size changed. That pairing — same shape, possibly different size — is what we are going to study carefully.
You already know one stronger idea from earlier classes. Two figures are congruent when they are exactly the same shape AND exactly the same size — like two ₹10 coins, or two pages torn from the same fresh notebook. Place one on the other and it covers it perfectly.
Similarity is the gentler cousin of congruence. We drop the "same size" demand and keep only "same shape".
You can now say, in your own words, the one-line difference: congruent means same shape and same size, similar means same shape, any size.