VSEPR told you the shape; this is its cause. A carbon's bare orbitals — one round 2s and three perpendicular 2p — point the wrong way to make four equal bonds at 109.5°. So they mix: one s plus three p recombine into four identical sp³ hybrids aimed at the corners of a tetrahedron. Use fewer p's and you get fewer, wider hybrids — sp² at 120°, sp at 180° — each leaving spare p orbitals behind that become π bonds. This is the engine under the last two rungs: water's oxygen is sp³ (the 104.5°), CO₂'s carbon is sp (the linear, plus two π's).
| sp³ | O in H₂O → the 104.5° bend; C in CH₄ |
| sp² | C=C ethene, BF₃ → 120°, 1 π |
| sp | C in CO₂, C≡C → 180°, 2 π |