Then insert a pin or nail. If you need to keep the windows open for air, make the opening small enough so the average adult can't climb through. If your budget permits, key locks also work well and are readily available for all types of windows. There are many door locks on the market today. Obviously, you want one that's hard to pick.
Experts recommend the pin cylinder lock.
And, better yet, a deadbolt. Most dead bolts are opened from the, out side with a key and a thumb turn on the inside. If your door is flanked by glass panels, a determined prowler can easily break the glass to access the thumb turn. If area fire regulations permit, it's best to purchase deadbolts that require a key on either side. Ask your hardware dealer or locksmith for the name of a reputable brand.