diff --git a/water.js b/water.js
index 8e002a8..36a9385 100644
--- a/water.js
+++ b/water.js
@@ -38,12 +38,23 @@ function follow_mouse(event) {
   var mouse_x = event.clientX
   var mouse_y = event.clientY
 
-  // convert screen coordinates to grid coordinates
-  var grid_x = (mouse_x - grid_margin - offset_h).div(grid_size);
-  var grid_y = (mouse_y - grid_margin - offset_v).div(grid_size);
+  // find the screen-space coordinates of the nearest grid cell
+  var mouse_x_within_grid = mouse_x - offset_h
+  var mouse_y_within_grid = mouse_y - offset_v
+  var cell_center_x = mouse_x_within_grid - (grid_margin/2) - (grid_size/2)
+  var cell_center_y = mouse_y_within_grid - (grid_margin/2) - (grid_size/2)
+
+  // divide and round to get the cell coordinates
+  var grid_x = Math.round(cell_center_x / grid_size)
+  var grid_y = Math.round(cell_center_y / grid_size)
 
   // draw a square there
-  draw_grid_square(grid_y, grid_x, 'white');
+  water_cells.push({
+    row: grid_x,
+    col: grid_y,
+    energy: 1.0
+  })
+  //draw_grid_square(grid_y, grid_x, '#6485ef');
 }
 
 // update and draw all walkers in the array
@@ -86,6 +97,7 @@ function add_random_direction(walker) {
  ****************************************************************************/
 
 // from https://www.sitepoint.com/javascript-generate-lighter-darker-color/
+// lum: -0.1 is 10% darker, 0.2 is 20% brighter
 function tweak_color_luminance(hex, lum) {
   // validate hex string
   hex = String(hex).replace(/[^0-9a-f]/gi, '');
@@ -152,6 +164,12 @@ var n_cols = canvas.width.div(grid_size);
 var offset_h = (canvas.width - (grid_size * n_cols)).div(2);
 var offset_v = (canvas.height - (grid_size * n_rows)).div(2);
 
+// array of cells. each one will have
+//   - row and column of the grid
+//   - energy (0 to 1)
+//   - TODO list add others if I make it more complex
+var water_cells[];
+
 // away we go!
 init();