The O(3) nonlinear sigma model in 2D describes a field φ(x,y) constrained to the unit sphere S² (3-component unit vector). Topological solitons (baby skyrmions) are stable field configurations with integer winding number Q = (1/4π)∫ φ·(∂ₓφ × ∂ᵧφ) d²x. The energy E ≥ 4π|Q| (Bogomolny bound). The exact Q=1 soliton (Belavin-Polyakov 1975) maps the plane to S² via stereographic projection. Q is conserved under smooth deformations.