void interactions() {
// KEYBOARD INTERACTIONS
  if (keyPressed) {   

    // RESTORE ALL
    if (key=='r') {
      for (int i=0; i<S; i++) {
        dens[i]    = dens0[i];
        s[i]       = 0;
        vx[i]      = vx0[i];
        vy[i]      = vy0[i];
        fx[i]      = 0;
        fy[i]      = 0;       
        isFluid[i] = true;
      }
      delay(100);
    }

    // GRID VISIBLE
    if (key=='g') { 
      gridVisible = !gridVisible; 
      delay(100);
    }

    // DENSITY FIELD VISIBLE
    if (key=='d') { 
      densityVisible = !densityVisible; 
      delay(100);
    }

    // VELOCITY FIELD VISIBLE
    if (key=='v') { 
      velocityVisible = !velocityVisible; 
      delay(100);
    }

    // PARTICLES VISIBLE
    if (key=='p') { 
      particlesVisible = !particlesVisible; 
      delay(100);
    }
    
    // HELP TEXT VISIBLE
    if (key=='h') { 
      helpVisible = !helpVisible; 
      delay(100);
    }
    
    // ADD SOURCE
    if (key=='s') {
      int x = int(mouseX/h);
      int y = int(mouseY/h);
      int size = penSize/2;
      for (int i=x-size; i<x+size; i++) {
        for (int j=y-size; j<y+size; j++) {
          if (i>0 && i<N+2 && j>0 && j<N+2) {
            s[IX(i, j)] = 1000;
          }
        }
      }
    }

    // DO SOMETHING
    if (key=='q') {
      int x = int(mouseX/h);
      int y = int(mouseY/h);
      int size = 10;
      for (int i=x-size; i<x+size; i++) {
        for (int j=y-size; j<y+size; j++) {
          if (i>0 && i<N+2 && j>0 && j<N+2) {
            isFluid[IX(i,j)] = false;
          }
        }
      }
    }
    
  }

  // MOUSE INTERACTION
  if (mousePressed) {
    int x = int(mouseX/h);
    int y = int(mouseY/h);
    // CHANGE VELOCITY FIELD
    if (mouseButton==LEFT) {
      for (int i=x-penSize/2; i<x+penSize/2; i++) {
        for (int j=y-penSize/2; j<y+penSize/2; j++) {
          if (i>0 && i<N+2 && j>0 && j<N+2) {
            vx[IX(i, j)] += (mouseX-pmouseX)*5;
            vy[IX(i, j)] += (mouseY-pmouseY)*5;
          }
        }
      }
    }
    // ADD DENSITY
    else {
      int size = penSize/2;
      for (int i=x-size; i<x+size; i++) {
        for (int j=y-size; j<y+size; j++) {
          if (i>0 && i<N+2 && j>0 && j<N+2) {
            dens[IX(i, j)] += 100;
          }
        }
      }
    }
  }

}