Similar

Two shapes are Similar if the only difference is size (and possibly the need to turn or flip one around). 
Resizing is the Key
If one shape can become another using Resizing (also called dilation, contraction, compression, enlargement or even expansion), then the shapes are Similar:

These Shapes are Similar! 
There may be Turns, Flips or Slides, Too!
Sometimes it can be hard to see if two shapes are Similar, because you may need to turn, flip or slide one shape as well as resizing it.
Examples
These shapes are all Similar:



Resized 
Resized and Reflected 
Resized and Rotated 
Why is it Useful?
When two shapes are similar, then:
 corresponding angles are equal, and
 the lines are in proportion.
This can make life a lot easier when solving geometry puzzles, as in this example:

Example: What is the missing length here? 


Notice that the red triangle has the same angles as the main triangle ...
... they both have one right angle, and a shared angle in the left corner 


In fact you could flip over the red triangle, rotate it a little, then resize it and it would fit exactly on top of the main triangle. So they are similar triangles.
So the line lengths will be in proportion, and we can calculate:
? = 80 × (130/127) = 81.9
(No fancy calculations, just common sense!) 
Congruent or Similar?
If you don't need to resize to make the shapes the same, they are Congruent.
So, if the shapes become the same:
When you ... 

Then the shapes are ... 
... only Rotate, Reflect and/or Translate 

Congruent 
... need to Resize 

Similar 
