Simplify the following text for a Grade 7 native English-speaking person.
Original: The Lisbon Treaty was signed by the EU member states on 13 December 2007 , and entered into force on 1 December 2009 .
Simplified: The Treaty of Lisbon was signed in 2007 between 27 European states that are members of the European Union ( EU ) . It became effective on December 1 , 2009 .