diff --git a/petri/src/lib.rs b/petri/src/lib.rs
index 66d85a8..d206072 100644
--- a/petri/src/lib.rs
+++ b/petri/src/lib.rs
@@ -3,7 +3,7 @@ use rand::prelude::*;
 pub const CHUNK_SIZE: usize = 32;
 
 #[derive(Default, Debug, PartialEq, Clone, Copy)]
-pub struct Cell(u16);
+pub struct Cell(pub u16);
 
 #[derive(Debug)]
 pub struct Dish {
@@ -172,6 +172,14 @@ impl RulePattern {
 		}
 	}
 
+	pub fn get_mut(&mut self, x: usize, y: usize) -> Option<&mut Cell> {
+		if x >= self.width || y >= self.height {
+			None
+		} else {
+			self.contents[x + self.width * y].as_mut()
+		}
+	}
+
 	pub fn height(&self) -> usize {
 		self.height
 	}
diff --git a/uscope/src/main.rs b/uscope/src/main.rs
index b9c2a80..796f3a1 100644
--- a/uscope/src/main.rs
+++ b/uscope/src/main.rs
@@ -1,5 +1,5 @@
 use eframe::{
-	egui::{CentralPanel, Color32, Painter, Rect, SidePanel, Ui, Vec2},
+	egui::{CentralPanel, Color32, Painter, Rect, Sense, SidePanel, Ui, Vec2},
 	NativeOptions,
 };
 use petri::{Chunk, Dish, Rule, CHUNK_SIZE};
@@ -92,15 +92,23 @@ fn rule_editor(ui: &mut Ui, rule: &mut Rule, cells: &[CellData]) {
 				bounds.min + Vec2::from((x as f32, y as f32)) * CSIZE,
 				Vec2::splat(CSIZE),
 			);
-			if let Some(cell) = rule.from.get(x, y) {
+			if let Some(cell) = rule.from.get_mut(x, y) {
 				let color = cells[cell.id()].color;
 				ui.painter().rect(rect, 2., color, OUTLINE);
+				let a = ui.allocate_rect(rect, Sense::click());
+				if a.clicked() {
+					cell.0 = (cell.0 + 1) % cells.len() as u16;
+				}
 			}
 
-			if let Some(cell) = rule.to.get(x, y) {
+			if let Some(cell) = rule.to.get_mut(x, y) {
 				let rect = rect.translate(Vec2::X * (patt_width as f32 + 1.) * CSIZE);
 				let color = cells[cell.id()].color;
 				ui.painter().rect(rect, 2., color, OUTLINE);
+				let a = ui.allocate_rect(rect, Sense::click());
+				if a.clicked() {
+					cell.0 = (cell.0 + 1) % cells.len() as u16;
+				}
 			}
 		}
 	}