Question

why water changes into water-vapour without boiling

Answer 1

This process is called evaporation.

http://en.wikipedia.org/wiki/Evaporation
http://www.weatherquestions.com/What_is_evaporation.htm
http://www.newton.dep.anl.gov/askasci/chem03/chem03444.htm
http://www.encyclopedia.com/doc/1E1-evaporat.html

I hope this helps. Thanks for asking.