This new cat of mine is a real Houdini.

Last night he jumped up onto the window sill over my bed where I often
put a fan with the window open wide enough for the fan to pull in cool
air.  He knocked the fan off and then pushed the screen out in one
corner and jumped down to the ground outside for a night on the town.

I wasn't too keen about his roaming around out there all night but
before I adopted him he was an outdoor only cat in the neighborhood so
I figured he'd come back in the morning for breakfast, which he did.

I was telling a friend about this and she was telling me about how she
was babysitting a neighbor's cats and one of them tore a hole in a
screen and escaped.

I'm a fresh air nut and like to keep my windows open as much as
possible.

I'm trying to think of ways of securing my place so he can't get out
the windows.  One idea I have is to put something on the window sills
that would discourage him from getting on them.  For example, something
with soft spikes that would be uncomfortable but wouldn't hurt him.  My
sills are about 4 inches deep.

I'm hoping others might have some good suggestions.