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The question is "Why do you need to?".
If you do, you'll need Spatial Analyst or 3D Analyst.
I suppose you could convert the polylines to points and then interpolate a surface using Spatial, or create a TIN from the contour lines using 3D.
Elizabeth,
A bounding polyline that intersects the open contours will probably do the job. However, you may find that some contour lines are discontinuous and need to be connected or merged. You do not need Spatial Analyst or any extensions, just the basic ArcView or whatever it is called in 10.1 unless you would like to make a raster image using the contours, which requires Spatial Analyst.
I'm not familiar with the 10.1 Construct Polygons tool, but it probably works along the same lines as the procedures that work in ArcView 9.x and 10 using "Auto-Complete Polygon" described in some of my other posts including some involving contour maps with links below.
http://forums.arcgis.com/threads/6786-polyline-to-polygon?highlight=Hardolph
http://forums.arcgis.com/threads/5189-how-to-create-polygons-from-digitized-contour-polylines?highli...
In all cases you need closed polylines and I recommend copying all the lines of interest to a "scratch" file where you merge them as the basis for forming the polygons. The lines can be copied from many different layers so your bounding polygon can be a separate feature class/shapefile.
Hardolph