ArrayList of ArrayList and unchecked conversions

I seem to be having some trouble with a list of lists. Here's the
code:

 //build a list of lists
 ArrayList<ArrayList> listOfLists = new ArrayList<ArrayList>();

 //build a list of DataPoints
 ArrayList<DataPoint> listOfPoints = new ArrayList<DataPoint>();

 //put a few DataPoints in the listOfPoints
 DataPoint point = new DataPoint("Howdy", 1);
 listOfPoints.add(point);
 point = new DataPoint("Dude", 2);
 listOfPoints.add(point);

 //put the listOfPoints in the listOfLists
 listOfLists.add(listOfPoints);

 //build a second list of DataPoinsts and put that in the listOfLIsts
 listOfPoints = new ArrayList<DataPoint>();
 point = new DataPoint("How are you", 20);
 listOfPoints.add(point);
 listOfLists.add(listOfPoints);

 //get the first element from each listOfPoints stored in the
listOfLists
 System.out.println(listOfLists.get(0).get(0)); // both work fine
 System.out.println(listOfLists.get(1).get(0));

 //try to retrieve the first list of data from the listOfLists
 listOfPoints = listOfLists.get(0); // ??? causes an unchecked
conversion
 System.out.println(listOfPoints.get(0)); //works fine

 //try to get the DataPoint directly from the listOfLists
 DataPoint retrievedPoint =
      listOfLists.get(0).get(0); // ??? causes "incompatible
type"

//----DataPoint Class Definition in seperate file
public class DataPoint
{
   public Object data;
   public int time;

   public DataPoint ()
   {
      data = null;
      time = 0;
   }

   public DataPoint(Object data, int ticks)
   {
      this.data = data;
      this.time = ticks;
   }

   public String toString()
   {
      String s = new String(data.toString() + " " + time);
      return s;
   }
}

The questions are these:
1) Why does the first ??? line cause the unchecked conversion?
1a) If that code isn't the right way to retrieve one of the lists
stored in the listOfLists, what is?

2) Why do both System.out.println statements work fine but the
second ??? causes an "incompatible type"
2a) How do I go about retrieving data items from one of the sublists
properly?

Any help greatly appreciated!