diff --git a/game.js b/game.js
index 7427b62..3215aa2 100644
--- a/game.js
+++ b/game.js
@@ -4358,7 +4358,7 @@ function magic_shrink()
   prey = getPrey(biome, macro.height * macro.height * 100, true);
 
   macro.shrunkPrey = prey;
-  macro.shrunkPrey.mass /= 1000000;
+  macro.shrunkPrey.mod_property("mass", x => x/1e2)
 
   let line = describe("magic-shrink", prey, macro, false, flat);
   let linesummary = summarize(prey.sum(), false);
diff --git a/recursive-macro.js b/recursive-macro.js
index 8c2b960..a92fdb4 100644
--- a/recursive-macro.js
+++ b/recursive-macro.js
@@ -780,6 +780,18 @@ function defaultSumProperty(thing) {
   };
 }
 
+function defaultModProperty(thing) {
+  return function(prop, func) {
+    thing[prop] = func(thing[prop]);
+
+    for (var key in thing.contents) {
+      if (thing.contents.hasOwnProperty(key)) {
+        thing.contents[key].mod_property(prop, func);
+      }
+    }
+  };
+}
+
 function defaultAddContent(thing) {
   return function(name, min, max, count) {
     if (min == max) {
@@ -797,6 +809,7 @@ function DefaultEntity() {
   this.area = defaultArea;
   this.mass = defaultMass;
   this.sum_property = defaultSumProperty;
+  this.mod_property = defaultModProperty;
   this.merge = defaultMerge;
   this.multiply = defaultMultiply;
   this.describeSimple = defaultDescribeSimple;