diff --git a/gloss-examples.cabal b/gloss-examples.cabal
--- a/gloss-examples.cabal
+++ b/gloss-examples.cabal
@@ -1,5 +1,5 @@
 Name:                gloss-examples
-Version:             1.7.3.1
+Version:             1.7.3.2
 License:             MIT
 License-file:        LICENSE
 Author:              Ben Lippmeier
diff --git a/raster/Fluid/UserEvent.hs b/raster/Fluid/UserEvent.hs
--- a/raster/Fluid/UserEvent.hs
+++ b/raster/Fluid/UserEvent.hs
@@ -10,34 +10,26 @@
 -- | Handle user events for the Gloss `playIO` wrapper.
 userEvent :: Config -> Event -> Model -> Model
 userEvent config
-        (EventKey key keyState _mods (x, y)) 
+        (EventKey key keyState mods (x, y)) 
         (Model df ds vf vs cl sp _cb)
 
-        | MouseButton G.LeftButton <- key
+        -- Add velocity ---------------------------------------------
+        | MouseButton G.RightButton     <- key
         , Down                          <- keyState
-        , (x',y')                       <- windowToModel config (x, y) 
-        = Model { densityField   = df
-                , densitySource  = Just (Source (Z:.y':.x') 1)
-                , velocityField  = vf
-                , velocitySource = vs
-                , clickLoc       = cl
-                , stepsPassed    = sp
-                , currButton     = M.LeftButton
-                }
-
-        | MouseButton G.LeftButton <- key
-        , Up                       <- keyState
+        , (x',y')                       <- windowToModel config (x,y)
         = Model { densityField   = df
                 , densitySource  = ds
                 , velocityField  = vf
                 , velocitySource = vs
-                , clickLoc       = cl
+                , clickLoc       = Just (x',y')
                 , stepsPassed    = sp
-                , currButton     = M.None
+                , currButton     = M.RightButton
                 }
 
-        | MouseButton G.RightButton     <- key
+        -- Accept shift-leftbutton for people with trackpads
+        | MouseButton G.LeftButton      <- key
         , Down                          <- keyState
+        , Down                          <- shift mods
         , (x',y')                       <- windowToModel config (x,y)
         = Model { densityField   = df
                 , densitySource  = ds
@@ -62,13 +54,39 @@
                  , currButton     = M.None
                  }
 
-        -- Reset model
+
+        -- Add density ----------------------------------------------
+        | MouseButton G.LeftButton <- key
+        , Down                          <- keyState
+        , (x',y')                       <- windowToModel config (x, y) 
+        = Model { densityField   = df
+                , densitySource  = Just (Source (Z:.y':.x') 1)
+                , velocityField  = vf
+                , velocitySource = vs
+                , clickLoc       = cl
+                , stepsPassed    = sp
+                , currButton     = M.LeftButton
+                }
+
+        | MouseButton G.LeftButton <- key
+        , Up                       <- keyState
+        = Model { densityField   = df
+                , densitySource  = ds
+                , velocityField  = vf
+                , velocitySource = vs
+                , clickLoc       = cl
+                , stepsPassed    = sp
+                , currButton     = M.None
+                }
+
+
+        -- Reset model ----------------------------------------------
         | Char 'r' <- key
         , Down     <- keyState
         = initModel (configInitialDensity config)
                     (configInitialVelocity config)
 
-        -- Quit program
+        -- Quit program ---------------------------------------------
         | Char 'q' <- key
         , Down     <- keyState
         = error "Quitting"