How can I lock my door without making any permanent changes?

I want to lock my door but I cant drill holes in it or the frame to insert a simple ';bar lock.'; Any other ideas? And how hard and expensive would it be to dissasemble my doorknob and attach one with a built in lock???How can I lock my door without making any permanent changes?
You have a couple of options, first you can put a doorknob with a lock like on your houses front door on it - that one has a key that you use from outside. Second you can put a doorknob with a lock on the inside like your bathroom. You can get either for about $15.00 and put them on using only a Phillips head screwdriver in about 10 minutesHow can I lock my door without making any permanent changes?
Its cheap and simple enough - check on the net, I'm sure some bloke with a big chin called Steve or something has posted instructions...
iam assuming this is an interior door so what you need to do is go pick up a lock-set that is labeled interior privatcy just like a bathroom door what you have on it now is called a passage knob you can get one for about $ 15.00 dollars and it takes 10 min to install
My Mom got me a kick bar that props under the door knob and has a ribbed foot to grab the floor. I tell you what, no body is getting in with that in place. I even take it on road trips to use in the hotels. Love it....
changing a door knob is simple. I'm a 65 yr old woman and just did it last week. And they are cheap. Just get one with that built in lock you mentioned for less than 20 bucks.
From the inside wedge a chair under the knob.


Un screw the screws and replace with a locking knob set. It's so easy I can do it!!!!