diff --git a/src/shaders/sandPhysics.frag b/src/shaders/sandPhysics.frag
new file mode 100644
index 0000000..a6df7f9
--- /dev/null
+++ b/src/shaders/sandPhysics.frag
@@ -0,0 +1,116 @@
+precision highp float;
+precision highp int;
+
+in vec2 vUv;
+
+uniform sampler2D uWorld;
+uniform sampler2D uMaterialProps;
+uniform vec2 uBlockOffset;
+uniform int uFrame;
+
+out vec4 fragColor;
+
+uint hash(uint x) {
+    x ^= x >> 16u;
+    x *= 0x45d9f3bu;
+    x ^= x >> 16u;
+    return x;
+}
+
+void main() {
+    ivec2 pixel = ivec2(gl_FragCoord.xy);
+    ivec2 blockBase = ((pixel - ivec2(uBlockOffset)) / 2) * 2 + ivec2(uBlockOffset);
+    ivec2 localPos = pixel - blockBase;
+    int localIndex = localPos.x + localPos.y * 2;
+
+    // bounds check
+    int worldSize = int(WORLD_SIZE);
+    if (blockBase.x < 0 || blockBase.y < 0 ||
+        blockBase.x + 1 >= worldSize || blockBase.y + 1 >= worldSize) {
+        fragColor = texelFetch(uWorld, pixel, 0);
+        return;
+    }
+
+    // read 2x2 block
+    vec4 cells[4];
+    float behaviors[4];
+    float densities[4];
+    for (int i = 0; i < 4; i++) {
+        ivec2 p = blockBase + ivec2(i % 2, i / 2);
+        cells[i] = texelFetch(uWorld, p, 0);
+        vec4 props = texelFetch(uMaterialProps, ivec2(int(cells[i].r), 0), 0);
+        behaviors[i] = props.r;
+        densities[i] = props.g;
+    }
+
+    // capture blocked state before vertical swaps modify the arrays
+    bool col0Blocked = (behaviors[0] == BEHAVIOR_SOLID || behaviors[2] == BEHAVIOR_SOLID || densities[0] <= densities[2]);
+    bool col1Blocked = (behaviors[1] == BEHAVIOR_SOLID || behaviors[3] == BEHAVIOR_SOLID || densities[1] <= densities[3]);
+
+    // random seed for this block this frame
+    uint seed = hash(uint(blockBase.x) * 7919u + uint(blockBase.y) * 6271u + uint(uFrame) * 3571u);
+
+    // try vertical gravity: column 0 (indices 0, 2) and column 1 (indices 1, 3)
+    // top=0,1  bottom=2,3
+    // block layout:
+    // [0][1]  <- top row
+    // [2][3]  <- bottom row
+    // if top is movable and top.density > bottom.density, swap
+
+    for (int col = 0; col < 2; col++) {
+        int top = col;      // 0 or 1
+        int bot = col + 2;  // 2 or 3
+
+        if (behaviors[top] != BEHAVIOR_SOLID && behaviors[bot] != BEHAVIOR_SOLID) {
+            if (densities[top] > densities[bot]) {
+                // swap entire vec4
+                vec4 tmp = cells[top];
+                cells[top] = cells[bot];
+                cells[bot] = tmp;
+                // update cached props
+                float tmpB = behaviors[top]; behaviors[top] = behaviors[bot]; behaviors[bot] = tmpB;
+                float tmpD = densities[top]; densities[top] = densities[bot]; densities[bot] = tmpD;
+            }
+        }
+    }
+
+    // try diagonal: if top cell still has high density and couldn't fall straight
+    // for top-left (0): can it go diagonal to bottom-right (3)?
+    // for top-right (1): can it go diagonal to bottom-left (2)?
+    // only if the cell directly below is blocked (same or higher density)
+    bool tryLeftFirst = (seed & 1u) == 0u;
+
+    if (tryLeftFirst) {
+        // try 0->3 diagonal (top-left to bottom-right)
+        if (col0Blocked && behaviors[0] != BEHAVIOR_SOLID && behaviors[3] != BEHAVIOR_SOLID &&
+            densities[0] > densities[3]) {
+            vec4 tmp = cells[0]; cells[0] = cells[3]; cells[3] = tmp;
+            float tmpB = behaviors[0]; behaviors[0] = behaviors[3]; behaviors[3] = tmpB;
+            float tmpD = densities[0]; densities[0] = densities[3]; densities[3] = tmpD;
+        }
+        // try 1->2 diagonal (top-right to bottom-left)
+        if (col1Blocked && behaviors[1] != BEHAVIOR_SOLID && behaviors[2] != BEHAVIOR_SOLID &&
+            densities[1] > densities[2]) {
+            vec4 tmp = cells[1]; cells[1] = cells[2]; cells[2] = tmp;
+            float tmpB = behaviors[1]; behaviors[1] = behaviors[2]; behaviors[2] = tmpB;
+            float tmpD = densities[1]; densities[1] = densities[2]; densities[2] = tmpD;
+        }
+    } else {
+        // try 1->2 first
+        if (col1Blocked && behaviors[1] != BEHAVIOR_SOLID && behaviors[2] != BEHAVIOR_SOLID &&
+            densities[1] > densities[2]) {
+            vec4 tmp = cells[1]; cells[1] = cells[2]; cells[2] = tmp;
+            float tmpB = behaviors[1]; behaviors[1] = behaviors[2]; behaviors[2] = tmpB;
+            float tmpD = densities[1]; densities[1] = densities[2]; densities[2] = tmpD;
+        }
+        // try 0->3
+        if (col0Blocked && behaviors[0] != BEHAVIOR_SOLID && behaviors[3] != BEHAVIOR_SOLID &&
+            densities[0] > densities[3]) {
+            vec4 tmp = cells[0]; cells[0] = cells[3]; cells[3] = tmp;
+            float tmpB = behaviors[0]; behaviors[0] = behaviors[3]; behaviors[3] = tmpB;
+            float tmpD = densities[0]; densities[0] = densities[3]; densities[3] = tmpD;
+        }
+    }
+
+    fragColor = cells[localIndex];
+}
diff --git a/src/shaders/sandPhysics.vert b/src/shaders/sandPhysics.vert
new file mode 100644
index 0000000..901d04b
--- /dev/null
+++ b/src/shaders/sandPhysics.vert
@@ -0,0 +1,12 @@
+precision highp float;
+precision highp int;
+
+in vec3 position;
+in vec2 uv;
+
+out vec2 vUv;
+
+void main() {
+    vUv = uv;
+    gl_Position = vec4(position, 1.0);
+}