hahahaha, First Saco treat Seventh with respect, hes our top moderator.. but If you can't or don't want to read the tutorials here:
1. Make a brush textured with a glass texture, (make it thin.)
2. Now right click on the brush and select "Tie to Entity".
3. A dialog box will appear, where it says func_door (or what ever
you have it configured to..)click the small arrow, whereas you
should see a long list, Select "func_breakable"
4. Now set the properties for glass... These will not make the window invisible however... this just makes it so when its breaks there will be the sound of glass crashing and some glass will fly everywhere (you would'nt want wood flyin' everywhere when you broke glass...lol)
5. To make the window seethru you will have to set the:
-RenderMode to Texture
-FX Ammount to about 128 (1 is solid, 255 is invisible)
and walla, go and shoot the window,
With help on building a breakable glass house (greenhouse)
Further read the Glass Tutorial Here:
http://cariad.co.za/twhl/tutorial.php?id=11 Good Luck, And happy Mapping