Index: wp-content/themes/twentytwelve/style.css
===================================================================
--- wp-content/themes/twentytwelve/style.css	(revision 21952)
+++ wp-content/themes/twentytwelve/style.css	(working copy)
@@ -1435,6 +1435,12 @@
 		padding: 0.571428571rem 0.714285714rem;
 		width: 120px;
 		width: 8.571428571rem;
+		-moz-hyphens: auto;
+		-ms-word-break: break-all;
+		-webkit-hyphens: auto;
+		hyphens: auto;
+		word-break: break-all;
+		word-wrap: break-word;
 	}
 	.main-navigation li ul li a:hover {
 		background: #e3e3e3;