diff --git a/CHANGELOG.md b/CHANGELOG.md
index fcabaa8..ff5a51b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,8 +4,8 @@
 
 ### Added
 
-- `Fluid` now has a new parameter `InteractionGroup`,
-  which controls which static particles (`Boundary`s) can interact with it.
+- `Fluid` and `Boundary`s now have a new property `interaction_groups`,
+  which controls which other particles can interact with it.
 
 ### Changed
 
diff --git a/examples2d/layers2.rs b/examples2d/layers2.rs
index c7c9d34..20155aa 100644
--- a/examples2d/layers2.rs
+++ b/examples2d/layers2.rs
@@ -78,7 +78,12 @@ pub fn init_world(testbed: &mut Testbed) {
     plugin.set_fluid_color(fluid_handle, Point3::new(1.0, 0.4, 0.6));
 
     let viscosity = ArtificialViscosity::new(0.5, 0.0);
-    let mut fluid = Fluid::new(points3, PARTICLE_RADIUS, 1.0, InteractionGroups::none());
+    let mut fluid = Fluid::new(
+        points3,
+        PARTICLE_RADIUS,
+        1.0,
+        InteractionGroups::new(Group::GROUP_2, Group::GROUP_2),
+    );
     fluid.nonpressure_forces.push(Box::new(viscosity.clone()));
     let fluid_handle = fluids_pipeline.liquid_world.add_fluid(fluid);
     plugin.set_fluid_color(fluid_handle, Point3::new(0.6, 0.8, 0.5));
diff --git a/src/geometry/contacts.rs b/src/geometry/contacts.rs
index ce6b5fa..a4d0964 100644
--- a/src/geometry/contacts.rs
+++ b/src/geometry/contacts.rs
@@ -273,7 +273,9 @@ fn compute_contacts_for_pair_of_cells(
                         HGridEntry::BoundaryParticle(boundary_j, particle_j) => {
                             let bi = &boundaries[*boundary_i];
                             let bj = &boundaries[*boundary_j];
-                            if !bi.fluid_interaction.test(bj.fluid_interaction) {
+                            if *boundary_i != *boundary_j
+                                && !bi.interaction_groups.test(bj.interaction_groups)
+                            {
                                 continue;
                             }
 
@@ -311,7 +313,7 @@ fn compute_contacts_for_pair_of_cells(
                             }
                             let bi = &boundaries[*boundary_i];
                             let fj = &fluids[*fluid_j];
-                            if !bi.fluid_interaction.test(fj.boundary_interaction) {
+                            if !bi.interaction_groups.test(fj.interaction_groups) {
                                 continue;
                             }
                             let pi = &boundaries[*boundary_i].positions[*particle_i];
@@ -343,13 +345,21 @@ fn compute_contacts_for_pair_of_cells(
                     let pj = if is_boundary_j {
                         let bj = &boundaries[fluid_j];
                         if !fluids[*fluid_i]
-                            .boundary_interaction
-                            .test(bj.fluid_interaction)
+                            .interaction_groups
+                            .test(bj.interaction_groups)
                         {
                             continue;
                         }
                         boundaries[fluid_j].positions[particle_j]
                     } else {
+                        if *fluid_i != fluid_j {
+                            let groups_i = &fluids[*fluid_i].interaction_groups;
+                            let groups_j = &fluids[fluid_j].interaction_groups;
+
+                            if !groups_i.test(*groups_j) {
+                                continue;
+                            }
+                        }
                         fluids[fluid_j].positions[particle_j]
                     };
 
diff --git a/src/object/boundary.rs b/src/object/boundary.rs
index 76a5c06..006bf95 100644
--- a/src/object/boundary.rs
+++ b/src/object/boundary.rs
@@ -19,13 +19,16 @@ pub struct Boundary {
     /// If this is set to `None` (which is the default), the boundary won't receive any
     /// force for fluids.
     pub forces: Option<RwLock<Vec<Vector<Real>>>>,
-    /// Determines which fluids this boundary is allowed to interact with.
-    pub fluid_interaction: InteractionGroups,
+    /// Determines which other particles is allowed to interact with.
+    pub interaction_groups: InteractionGroups,
 }
 
 impl Boundary {
     /// Initialize a boundary object with the given particles.
-    pub fn new(particle_positions: Vec<Point<Real>>, fluid_interaction: InteractionGroups) -> Self {
+    pub fn new(
+        particle_positions: Vec<Point<Real>>,
+        interaction_groups: InteractionGroups,
+    ) -> Self {
         let num_particles = particle_positions.len();
         let velocities = std::iter::repeat(Vector::zeros())
             .take(num_particles)
@@ -39,7 +42,7 @@ impl Boundary {
             velocities,
             volumes,
             forces: None,
-            fluid_interaction,
+            interaction_groups,
         }
     }
 
diff --git a/src/object/fluid.rs b/src/object/fluid.rs
index 5583547..69bb356 100644
--- a/src/object/fluid.rs
+++ b/src/object/fluid.rs
@@ -28,8 +28,9 @@ pub struct Fluid {
     num_deleted_particles: usize,
     /// The particles radius.
     particle_radius: Real,
-    /// The groups controlling which static particles ([`Boundary`]) can interact with this fluid.
-    pub boundary_interaction: InteractionGroups,
+    /// The groups controlling which other particles can interact with this fluid.
+    /// A fluid always interacts with itself.
+    pub interaction_groups: InteractionGroups,
 }
 
 impl Fluid {
@@ -40,7 +41,7 @@ impl Fluid {
         particle_positions: Vec<Point<Real>>,
         particle_radius: Real, // XXX: remove this parameter since it is already defined by the liquid world.
         density0: Real,
-        boundary_interaction: InteractionGroups,
+        interaction_groups: InteractionGroups,
     ) -> Self {
         let num_particles = particle_positions.len();
         let velocities: Vec<_> = std::iter::repeat(Vector::zeros())
@@ -58,7 +59,7 @@ impl Fluid {
             volumes: std::iter::repeat(particle_volume)
                 .take(num_particles)
                 .collect(),
-            boundary_interaction,
+            interaction_groups,
             deleted_particles: std::iter::repeat(false).take(num_particles).collect(),
             num_deleted_particles: 0,
             density0,