A big smiley face. :)

    uniform float time;
    varying vec2 vUv;

    // https://iquilezles.org/articles/distfunctions2d/ (Thanks IQ!)
    float sdRing( in vec2 p, in vec2 n, in float r, float th )
    {
        p.x = abs(p.x);
   
        p = mat2x2(n.x,n.y,-n.y,n.x)*p;

        return max( abs(length(p)-r)-th*0.5,
                length(vec2(p.x,max(0.0,abs(r-p.y)-th*0.5)))*sign(p.x) );
    }

    void main() {

        // create coordinates at visual centre (y coords doubled for circles)
        vec2 centre = vec2(0.25, 0.25);        
        vec2 uv = vUv * vec2(1.0, 0.5) - centre;

        // mirror x to get both eyes for the price of one, and find eye offset
        float eyes = length(vec2(abs(uv.x), uv.y) - vec2(0.035, 0.03));
        // carve them out using smoothstep
        eyes = smoothstep(0.015, 0.016, eyes);

        float mouth = sdRing(vec2(uv.x, -uv.y + 0.03), vec2(7), 0.65, 0.1);
        mouth = smoothstep(0.02, 0.025, mouth);

        float shade = min(eyes, mouth);
    
        vec3 yellow = vec3(0.9, 0.7, 0.0);
        vec3 color = shade * yellow;

        gl_FragColor = vec4(color, 1.0);
    }