diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..ed8ebf5
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+__pycache__
\ No newline at end of file
diff --git a/gas_sim.py b/gas_sim.py
index 5cc53cf..5f1c567 100644
--- a/gas_sim.py
+++ b/gas_sim.py
@@ -1,30 +1,23 @@
 from math import hypot
 
 
-class Particle:
-    def __init__(self, pos, vel):
-        self.pos = pos
-        self.vel = vel
-
-    def update(self, dt):
-        self.pos = (self.pos[0] + self.vel[0] * dt, self.pos[1] + self.vel[1] * dt)
-
-
 class GasSim:
-    def __init__(self, particle_radius, particle_compressibility, substeps):
+    def __init__(self, particle_radius, particle_compressibility, particle_friction, substeps):
         self.particle_radius = particle_radius
         self.particle_compressibility = particle_compressibility
+        self.particle_friction = particle_friction
         self.substeps = substeps
 
         self.particles = []
+        self.walls = []
 
     def add_particle(self, pos, vel = (0, 0)):
-        self.particles.append(Particle(pos, vel))
-
-    def return_particles_pos(self):
-        return tuple(particle.pos for particle in self.particles)
+        self.particles.append([pos, vel])
 
     def cursor_particle_collide_test(self, cursor_pos, cursor_radius):
         for particle in self.particles:
-            if hypot(particle.pos[0] - cursor_pos[0], particle.pos[1] - cursor_pos[1]) <= cursor_radius + self.particle_radius:
-                return True
\ No newline at end of file
+            if hypot(particle[0][0] - cursor_pos[0], particle[0][1] - cursor_pos[1]) <= cursor_radius + self.particle_radius:
+                return True
+            
+    def add_wall(self, start, end):
+        self.walls.append((start, end))
\ No newline at end of file
diff --git a/main.py b/main.py
index 4dbce2e..617eac1 100644
--- a/main.py
+++ b/main.py
@@ -1,15 +1,18 @@
 import pygame
 import pygame.gfxdraw
 import gas_sim as gs
+import numpy as np
 
 ##### CONFIG #####
 window_size = (1280, 720)
+target_fps = 60
+
+particle_radius_ws = 0.025
+particle_compressibility = 0.1
+particle_friction = 0.1
 
 
-particle_radius_ws = 0.1
-particle_compressibility = 0.5
-
-cursor_radius_ws = 0.1
+cursor_radius_ws = 0.15
 
 substeps = 10
 ##################
@@ -19,6 +22,7 @@ substeps = 10
 
 window_aspect = window_size[0] / window_size[1]
 cursor_radius_ss = cursor_radius_ws * window_size[1]
+particle_radius_ss = particle_radius_ws * window_size[1]
 
 
 # pygame init
@@ -27,9 +31,19 @@ window = pygame.display.set_mode(window_size)
 clock = pygame.time.Clock()
 
 
-
 # gas sim init
-gas_sim = gs.GasSim(particle_radius_ws, particle_compressibility, substeps)
+gas_sim = gs.GasSim(particle_radius_ws, particle_compressibility, particle_friction, substeps)
+
+
+# add walls
+gas_sim.add_wall((0, 0), (window_aspect, 0))
+gas_sim.add_wall((0, 1), (window_aspect, 1))
+gas_sim.add_wall((0, 0), (0, 1))
+gas_sim.add_wall((1, 0), (1, 1))
+
+
+not_loaded_particles = True
+dt = target_fps / 1000
 
 
 
@@ -50,9 +64,35 @@ while True:
         mouse_pos_ws = (mouse_pos_ss[0] / window_size[1], mouse_pos_ss[1] / window_size[1])
 
         # if not colliding with any particles
-        if not gas_sim.cursor_particle_collide_test(mouse_pos_ws, cursor_radius_ws):
+        if not gas_sim.cursor_particle_collide_test(mouse_pos_ws, particle_radius_ws):
             gas_sim.add_particle(mouse_pos_ws)
 
+    # on s key press
+    if pygame.key.get_pressed()[pygame.K_s]:
+        # save particles to file
+        particles = tuple(pos for pos, _ in gas_sim.particles)
+        np.save("particles.npy", particles)
+
+        print("Saved particles to file")
+    
+    # on l key press
+    if not_loaded_particles and pygame.key.get_pressed()[pygame.K_l]:
+        # load particles from file
+        particles = np.load("particles.npy")
+        for particle in particles:
+            gas_sim.add_particle(particle)
+
+        print("Loaded particles from file")
+
+        not_loaded_particles = False
+
+
+
+    # update particles
+    #gas_sim.update(dt)
+    
+
+
 
     window.fill((0, 0, 0))
 
@@ -61,11 +101,11 @@ while True:
     pygame.gfxdraw.aacircle(window, cursor_pos[0], cursor_pos[1], int(cursor_radius_ss), (255, 255, 255))
 
     # draw particles
-    for particle_pos in gas_sim.return_particles_pos():
+    for particle_pos, _ in gas_sim.particles:
         particle_pos_ss = (int(particle_pos[0] * window_size[1]), int(particle_pos[1] * window_size[1]))
-        pygame.gfxdraw.aacircle(window, particle_pos_ss[0], particle_pos_ss[1], int(cursor_radius_ss), (255, 255, 255))
+        pygame.gfxdraw.aacircle(window, particle_pos_ss[0], particle_pos_ss[1], int(particle_radius_ss), (255, 255, 255))
 
 
     # update
     pygame.display.update()
-    clock.tick(60)
\ No newline at end of file
+    dt = clock.tick(60) / 1000
\ No newline at end of file
diff --git a/particles.npy b/particles.npy
new file mode 100644
index 0000000..cf35a8f
Binary files /dev/null and b/particles.npy differ