Such posts make me rethink everything I know about my body and, surprisingly, they help with maintaing my self-esteem on its level. Seeing such mundane things as "walking on two legs" appearing as wonderful is really wholesome.
Centaurism is when over many generations, a pair of front limbs goes from being used for support and locomotion to being permanently held off the ground for grasping, object manipulation, or other purposes. Basically turning a pair of front feet into a pair of hands.
Having four legs is often considered to be the ideal number for large animals, being very stable and resource efficient.
Therefore, it stands to reason that intelligent aliens with six limbs (4 legs, 2 hands) or more would be very common, and tool use on planets where the equivalent to Earth's vertebrates have 6 legs would be much more common among species than it is on Earth.
Aliens would be astounded by our ability to balance on merely two feet and our incredible flexibility. They would also be fascinated that some species of birds of all things figured out how to use tools.
"How did your raven use the key card?! Its front limbs are wings! Its back limbs are legs! It doesn't have hands!"
"She used her mouth."
"Ohhhhhhhhhh... Amazing."