when i choose in form selection "other" must appear new text field.

e.g.

<select name=brand>
<option value=1>acer</option>
<option value=2>toshiba</option>
<option value=3>other</option>
</select>
<input type=text name=other_brand /> //hide until i choose other option

<select name=cpu>
<option value=1>intel</option>
<option value=2>athlon</option>
<option value=3>other</option>
</select>
<input type=text name=other_cpu /> //hide until i choose other option