execute <body> sections when page loading,and <head> sections will be executed when call it.
External script cannot contain the <script></script> tags
To use an external script,point to the .js file in the "src" attribute of the <script> tag.
Alet Box: An alert box is often used if you want to make sure information comes through to the user.When an alert box pops up,the user will have to click "OK" to proceed. alert("sometext");
The for...in statement loops through the properties of an object