diff --git a/main.cpp b/main.cpp
index fd166b4..594d787 100644
--- a/main.cpp
+++ b/main.cpp
@@ -2,16 +2,16 @@
 #include <cstring>
 #include "simulation.h"
 
-void printGrid(simulation::SimulationGrid grid)
+void printGrid(simulation::SimulationGrid* grid)
 {
-    std::cout << std::string(grid.x_max+2, '_') << std::endl;
-    for (unsigned char y = grid.y_max; y > 0; y--)
+    std::cout << std::string(grid->x_max+2, '_') << std::endl;
+    for (unsigned char y = grid->y_max; y > 0; y--)
     {
         std::cout << "|";
-        for (unsigned char x = 0; x < grid.x_max; x++)
+        for (unsigned char x = 0; x < grid->x_max; x++)
         {
-            unsigned int offset = simulation::getCellOffset(x, y-1, 0, grid);
-            unsigned int material = grid.out[offset];
+            unsigned int offset = getCellOffset(x, y-1, 0, grid);
+            unsigned int material = grid->out[offset];
             switch (material)
             {
             case simulation::VACUUM:
@@ -26,12 +26,12 @@ void printGrid(simulation::SimulationGrid grid)
         }
         std::cout << "|" << std::endl;
     }
-    std::cout << std::string(grid.x_max+2, '_') << std::endl;
+    std::cout << std::string(grid->x_max+2, '_') << std::endl;
 }
 
-void initGrid(simulation::SimulationGrid grid) {
-    unsigned int offset = simulation::getCellOffset(0,1,0, grid);
-    grid.in[offset] = simulation::WATER;
+void initGrid(simulation::SimulationGrid* grid) {
+    unsigned int offset = simulation::getCellOffset(0,2,0, grid);
+    grid->in[offset] = simulation::WATER;
 }
 
 int main() {
@@ -50,11 +50,11 @@ int main() {
         zMax
     };
 
-    initGrid(grid);
-    step(0, grid);
-
+    initGrid(&grid);
     for (int n=0; n< 2; n++) {
-        printGrid(grid);
+        simulation::step(0, &grid);
+        printGrid(&grid);
+        prepareForNextStep(&grid);
     }
     std::free(inCells);
     std::free(outCells);
diff --git a/simulation.cpp b/simulation.cpp
index f26edb4..c06b317 100644
--- a/simulation.cpp
+++ b/simulation.cpp
@@ -1,16 +1,17 @@
 #include "simulation.h"
+#include <cstring>
 
 namespace simulation
 {
-    unsigned int getCellOffset(
+    int getCellOffset(
         unsigned char x,
         unsigned char y,
         unsigned char z,
-        SimulationGrid grid)
+        SimulationGrid* grid)
     {
-        if (x < grid.x_max && y < grid.y_max && z < grid.z_max)
+        if (x < grid->x_max && y < grid->y_max && z < grid->z_max)
         {
-            unsigned int offset = x + y * grid.x_max + z * (grid.y_max * grid.x_max);
+            int offset = x + y * grid->x_max + z * (grid->y_max * grid->x_max);
             return offset;
         }
         return -1;
@@ -18,33 +19,55 @@ namespace simulation
 
     void step(
         double millisecondsSinceLastStep,
-        SimulationGrid grid)
+        SimulationGrid* grid)
     {
-        for (unsigned char x = 0; x < grid.x_max; x++)
+        for (unsigned char x = 0; x < grid->x_max; x++)
         {
-            for (unsigned char y = 0; y < grid.y_max; y++)
+            for (unsigned char y = 0; y < grid->y_max; y++)
             {
-                for (unsigned char z = 0; z < grid.z_max; z++)
+                for (unsigned char z = 0; z < grid->z_max; z++)
                 {
-                    unsigned int offset = getCellOffset(x, y, z, grid);
-                    unsigned int material = grid.in[offset];
+                    int offset = getCellOffset(x, y, z, grid);
+                    unsigned int material = grid->in[offset];
                     switch (material)
                     {
                     case WATER:
-                        if (y > 0)
                         {
-                            unsigned int downOffset = getCellOffset(x, y - 1, z, grid);
+                            int downOffset = getCellOffset(x, y - 1, z, grid);
                             // Flow down if cell is empty
-                            if (grid.out[downOffset] == 0)
+                            if (downOffset >= 0 && grid->out[downOffset] == 0)
                             {
-                                grid.out[downOffset] = material;
+                                grid->out[downOffset] = material;
+                                grid->out[offset] = VACUUM;
+                            } else
+                            {
+                                // Stay in the same location
+                                grid->out[offset] = material;
                             }
+                            break;
+                        }
+                    case MERCURY:
+                        {
+                            break;
                         }
                     default:
-                        grid.out[offset] = material;
+                        {
+                            grid->out[offset] = material;
+                        }
                     }
                 }
             }
         }
     }
+
+    void prepareForNextStep(SimulationGrid* grid)
+    {
+        // Flip the buffers
+        unsigned int* inCells = grid->in;
+        grid->in = grid->out;
+        grid->out = inCells;
+        // Clear out buffer
+        size_t bufferSizeInBytes = sizeof(unsigned int) * grid->x_max * grid->y_max * grid->z_max;
+        std::memset(grid->out, VACUUM, bufferSizeInBytes);
+    }
 }
diff --git a/simulation.h b/simulation.h
index 4c72ef5..35469b6 100644
--- a/simulation.h
+++ b/simulation.h
@@ -24,15 +24,16 @@ namespace simulation
 
     void step(
         double millisecondsSinceLastStep,
-        SimulationGrid grid
+        SimulationGrid* grid
     );
 
-    unsigned int getCellOffset(
+    int getCellOffset(
         unsigned char x,
         unsigned char y,
         unsigned char z,
-        SimulationGrid grid);
+        SimulationGrid* grid);
 
+    void prepareForNextStep(SimulationGrid* grid);
 }
 
 #endif //SIMULATION_H