diff --git a/2021/inputs/06.txt b/2021/inputs/06.txt
new file mode 100644
index 0000000..f98f9bf
--- /dev/null
+++ b/2021/inputs/06.txt
@@ -0,0 +1 @@
+1,4,2,4,5,3,5,2,2,5,2,1,2,4,5,2,3,5,4,3,3,1,2,3,2,1,4,4,2,1,1,4,1,4,4,4,1,4,2,4,3,3,3,3,1,1,5,4,2,5,2,4,2,2,3,1,2,5,2,4,1,5,3,5,1,4,5,3,1,4,5,2,4,5,3,1,2,5,1,2,2,1,5,5,1,1,1,4,2,5,4,3,3,1,3,4,1,1,2,2,2,5,4,4,3,2,1,1,1,1,2,5,1,3,2,1,4,4,2,1,4,5,2,5,5,3,3,1,3,2,2,3,4,1,3,1,5,4,2,5,2,4,1,5,1,4,5,1,2,4,4,1,4,1,4,4,2,2,5,4,1,3,1,3,3,1,5,1,5,5,5,1,3,1,2,1,4,5,4,4,1,3,3,1,4,1,2,1,3,2,1,5,5,3,3,1,3,5,1,5,3,5,3,1,1,1,1,4,4,3,5,5,1,1,2,2,5,5,3,2,5,2,3,4,4,1,1,2,2,4,3,5,5,1,1,5,4,3,1,3,1,2,4,4,4,4,1,4,3,4,1,3,5,5,5,1,3,5,4,3,1,3,5,4,4,3,4,2,1,1,3,1,1,2,4,1,4,1,1,1,5,5,1,3,4,1,1,5,4,4,2,2,1,3,4,4,2,2,2,3
diff --git a/2021/src/day06.rs b/2021/src/day06.rs
index 113ba49..f52b0dc 100644
--- a/2021/src/day06.rs
+++ b/2021/src/day06.rs
@@ -1,9 +1,63 @@
 use std::io::Read;
 
-pub fn part1(_input: &mut dyn Read) -> String {
-    todo!()
+fn fish_growth(fish: &[u8], days: usize) -> usize {
+    let mut fish_per_day = [0usize; 9];
+
+    for &life in fish {
+        fish_per_day[life as usize] += 1;
+    }
+
+    for day in 0..days {
+        let index = day % fish_per_day.len();
+        let offspring_today = fish_per_day[index];
+
+        // The parents can be parents in 6 days
+        fish_per_day[(index + 7) % fish_per_day.len()] += offspring_today;
+        // The offspring from today will be ready the next time they come around
+    }
+
+    fish_per_day.into_iter().sum()
 }
 
-pub fn part2(_input: &mut dyn Read) -> String {
-    todo!()
+fn part_common(input: &mut dyn Read, days: usize) -> String {
+    let mut buffer = String::new();
+    input.read_to_string(&mut buffer).unwrap();
+
+    let fish: Vec<u8> = buffer
+        .trim_end()
+        .split(',')
+        .map(|s| s.parse().unwrap())
+        .collect();
+
+    fish_growth(&fish, days).to_string()
+}
+
+pub fn part1(input: &mut dyn Read) -> String {
+    part_common(input, 80)
+}
+
+pub fn part2(input: &mut dyn Read) -> String {
+    part_common(input, 256)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: [u8; 5] = [3, 4, 3, 1, 2];
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!(fish_growth(&SAMPLE, 1), 5);
+        assert_eq!(fish_growth(&SAMPLE, 2), 6);
+        assert_eq!(fish_growth(&SAMPLE, 3), 7);
+        assert_eq!(fish_growth(&SAMPLE, 4), 9);
+        assert_eq!(fish_growth(&SAMPLE, 18), 26);
+        assert_eq!(fish_growth(&SAMPLE, 80), 5934);
+    }
+
+    #[test]
+    fn sample_part2() {
+        assert_eq!(fish_growth(&SAMPLE, 256), 26984457539);
+    }
 }