Help: have only 2 meshes, cannot select separately

Hi,

I am a newbee.

In my first scebe, I added one cylinder, and then one cube on top of it.

I want to select only the cube, but whatever I do, selects both.

I have read the manual. 

Neither RMB on the cube, nor any border selection that does not even
touch the cylinder, including the small ones completely within the
cube helped. I always end up selecting both.

Somehow I must have done something to tie them together, but I don't
know what.

Any ideas will be much appreciated.

Thanks