## Mathematica's user interface

Mathematica documents are often called "electronic notebooks"
and consist of a sequence of pairs of input and output **cells**.
A cell is where you enter a Mathematica command or receive output
from the program.

Open the Mathematica program on your computer. You should see
an empty notebook without any cells. Click the mouse somewhere
on the notebook to activate the cursor. Then type

**2 + 2**

and press the **Enter** key on the numeric keypad. You should
see a bracket appear at the right edge of the notebook window.
This bracket defines the extent of the input cell you just created.
Throughout this tutorial the indented lines represent Mathematica
input and output. Boldface lines will represent the input you
type into a Mathematica notebook. Mathematica automatically
puts your output into boldface, so your Mathematica notebook should
look very similar to the indented lines in this tutorial.
Pressing the **Enter** key instructs Mathematica to evaluate
the input cell that contains the cursor. (Make sure the cursor
is in an input cell, or else Mathematica won't do anything when
you press **Enter**.) In this case, we are asking Mathematica
to evaluate the expression 2 + 2. After some time, an output cell
will appear in your notebook with the result:

4

Notice that the input and output cells are labeled "In[1]" and
"Out[1]". Each has its own bracket, and they are grouped together
by an outer bracket.
It may seem that it took Mathematica a long time to perform
this simple calculation. This is because Mathematica loads
into your computer in two stages. When you open the Mathematica
icon, the user interface portion of the program is loaded. The
part of the program that actually does calculations doesn't get
loaded until the first time you press **Enter**.

You can use the mouse to copy text from a previous input cell
and paste it into the current input cell. This is useful if you
want to repeat a portion of an earlier calculation or if you
need to correct a spelling mistake. Just make sure the cursor
is in the current input cell before you press **Enter**.

Other parts of the Mathematica tutorial: