At this time of year, we all want to stay cool and get as much ventilation as possible. An open window or unlocked door is an invite to opportunist thieves and burglars.
Most modern windows have a facility to be locked in a ventilation position which will allow the flow of air but remain secure. When the window is open just have a look in the frame if you can see something like this.
If you can then you can open your window open a small amount and close the handle to lock it in position. Push the window to check it is locked. This will give you some much needed fresh air but remain secure.
Have a look at our windows here.