"android:elevation=" doesn't work on devices pre-Lollipop with compile API21

UPDATED ::

1. Best Practice to do that is

   <android.support.v7.widget.CardView>
       <YourLayout>
   </android.support.v7.widget.CardView>
   

   and add library for cardview

   dependencies {
      ...
      compile 'com.android.support:cardview-v7:21.0.+'
    }
   

2. On Pre-Lollipop you can use this drawable

   android:background="@android:drawable/dialog_holo_light_frame"

   it will give you the look of elevation

3. you can create your own like this

   <?xml version="1.0" encoding="utf-8"?>
   <layer-list xmlns:android="http://schemas.android.com/apk/res/android">
   
   <item>
   <shape android:shape="rectangle">
       <solid android:color="#BDBDBD"/>
       <corners android:radius="5dp"/>
   </shape>
   </item>
   
   <item
   android:left="0dp"
   android:right="0dp"
   android:top="0dp"
   android:bottom="2dp">
   <shape android:shape="rectangle">
       <solid android:color="#ffffff"/>
       <corners android:radius="5dp"/>
   </shape>
   </item>
   </layer-list>
   

reference

Elevation requires the device to run Lollipop. See this answer on how to simulate elevation https://stackoverflow.com/a/26747592/680249

You can also use a CardView from the support library to implement surfaces.
To do so add a dependency to your build.gradle:

compile 'com.android.support:cardview-v7:23.1.1'

And then simply use it in your layouts:

  <android.support.v7.widget.CardView
      android:layout_width="match_parent"
      android:layout_height="150dp"
      android:layout_margin="16dp"
      android:background="#fff"
      >
  </android.support.v7.widget.CardView>

Here you have a quite more options to customise it comparing to using @android:drawable/dialog_holo_light_frame as a background

EDIT:
Also notice, that this approach allows you to simply implement
Material Design on Pre-Lolipop devices.
You can change the elevation,
round the corners etc.
To do so you have to:

 app:cardElevation="8dp"
 app:cardCornerRadius="8dp"
 app:contentPadding="5dp">

And not forget to add xmlns:app="http://schemas.android.com/apk/res-auto" to the root layout.

Also you can easily change the elevation in your code:

CardView card = (CardView) findViewById(R.id.yourPreetyCoolCardView);
card.setCardElevation(getResources()
    .getDimension(R.dimen.card_picked_up_elevation));

Use 8dp for picked up and 2dp for resting (usual) state and you will be awesome.