--- a/debian-apt.rst	Sun May 29 19:26:03 2016 +0300
+++ b/debian-apt.rst	Sun May 29 22:12:07 2016 +0300
@@ -82,8 +82,8 @@
   $ netselect-apt experimental
   $ netselect-apt sid
 
-Package priority
-================
+Check package versions and priority
+===================================
 
 ``/etc/apt/preferences``::
 
@@ -286,3 +286,17 @@
   $ apt-rdepends -r $PKG
   $ apt-rdepends -d $PKG | dot -Tsvg >$PKG.svg && see $PKG.svg
   $ apt-rdepends -d -r $PKG | dot -Tsvg >$PKG.svg && see $PKG.svg
+
+Simulation mode
+===============
+
+Simulation for ``apt-get`` and ``aptitude`` shows what happen without actually
+modifying or installing any packages.
+
+Simulate install or upgrade with ``-s`` key::
+
+  $ sudo apt-get install -s xterm
+  $ sudo aptitude install -s xterm
+  $ sudo apt-get upgrade -s
+  $ sudo aptitude upgrade -s
+