diff --git a/main.cpp b/main.cpp
index 918249f..fd166b4 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->cells[offset];
+            unsigned int material = grid.out[offset];
             switch (material)
             {
             case simulation::VACUUM:
@@ -26,11 +26,15 @@ 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;
 }
 
-int main()
-{
+void initGrid(simulation::SimulationGrid grid) {
+    unsigned int offset = simulation::getCellOffset(0,1,0, grid);
+    grid.in[offset] = simulation::WATER;
+}
+
+int main() {
     unsigned char xMax = 50;
     unsigned char yMax = 20;
     unsigned char zMax = 1;
@@ -38,16 +42,20 @@ int main()
     auto inCells = static_cast<unsigned int*>(std::malloc(bufferSizeInBytes));
     std::memset(inCells, 0, bufferSizeInBytes);
     auto outCells = static_cast<unsigned int*>(std::malloc(bufferSizeInBytes));
-    simulation::SimulationGrid inGrid = {
-        inCells, xMax, yMax, zMax
-    };
-    simulation::SimulationGrid outGrid = {
-        outCells, xMax, yMax, zMax
+    simulation::SimulationGrid grid = {
+        inCells,
+        outCells,
+        xMax,
+        yMax,
+        zMax
     };
 
-    step(0, &inGrid, &outGrid);
+    initGrid(grid);
+    step(0, grid);
 
-    printGrid(&outGrid);
+    for (int n=0; n< 2; n++) {
+        printGrid(grid);
+    }
     std::free(inCells);
     std::free(outCells);
     return 0;
diff --git a/simulation.cpp b/simulation.cpp
index 3dd8fad..f26edb4 100644
--- a/simulation.cpp
+++ b/simulation.cpp
@@ -6,11 +6,11 @@ namespace simulation
         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);
+            unsigned int offset = x + y * grid.x_max + z * (grid.y_max * grid.x_max);
             return offset;
         }
         return -1;
@@ -18,31 +18,30 @@ namespace simulation
 
     void step(
         double millisecondsSinceLastStep,
-        SimulationGrid* input,
-        SimulationGrid* output)
+        SimulationGrid grid)
     {
-        for (unsigned char x = 0; x < input->x_max; x++)
+        for (unsigned char x = 0; x < grid.x_max; x++)
         {
-            for (unsigned char y = 0; y < input->y_max; y++)
+            for (unsigned char y = 0; y < grid.y_max; y++)
             {
-                for (unsigned char z = 0; z < input->z_max; z++)
+                for (unsigned char z = 0; z < grid.z_max; z++)
                 {
-                    unsigned int offset = getCellOffset(x, y, z, input);
-                    unsigned int material = input->cells[offset];
+                    unsigned 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, input);
+                            unsigned int downOffset = getCellOffset(x, y - 1, z, grid);
                             // Flow down if cell is empty
-                            if (output->cells[downOffset] == 0)
+                            if (grid.out[downOffset] == 0)
                             {
-                                output->cells[downOffset] = material;
+                                grid.out[downOffset] = material;
                             }
                         }
                     default:
-                        output->cells[offset] = material;
+                        grid.out[offset] = material;
                     }
                 }
             }
diff --git a/simulation.h b/simulation.h
index 5303ba3..4c72ef5 100644
--- a/simulation.h
+++ b/simulation.h
@@ -15,7 +15,8 @@ namespace simulation
 
     struct SimulationGrid
     {
-        unsigned int* cells;
+        unsigned int* in;
+        unsigned int* out;
         unsigned char x_max;
         unsigned char y_max;
         unsigned char z_max;
@@ -23,14 +24,14 @@ namespace simulation
 
     void step(
         double millisecondsSinceLastStep,
-        SimulationGrid* input,
-        SimulationGrid* output);
+        SimulationGrid grid
+    );
 
     unsigned int getCellOffset(
         unsigned char x,
         unsigned char y,
         unsigned char z,
-        SimulationGrid* grid);
+        SimulationGrid grid);
 
 }