This is part of a speech I wrote about computer freedom and free software. I think I read a similar comparison somewhere but I like to think that I made it all up by myself.
"Imagine you have recently bought a brand new bicycle for your little child (I find a little girl to be more heart-touching, that's why I refer to the child as "she"). She is very excited about the coming of spring because she'll be able to ride her shiny bike. On Sunday you're all ready to go to the park but then you find the bike has got a flat. No problem you say. I've got an old rusty pump in the garage, it's a just a matter of finding it. However when you are about to pump air into the wheel someone suddenly warns that you just can't do it because when you got the bicycle you signed a legal contract saying that you are not allowed to do that under any circumstances. You must take it to the technician who's the only person authorized to do that.
You will probably wonder: What? Why can't I?
Well, to a certain extent (and with several nuances) this is similar to what happens when you buy a computer with a legacy operating system. You own the machine because you have a ticket to prove that, but even if you have a ticket from the store for your legacy operating system you just can't say it is yours because in order to be able to use the software you are forced to accept certain terms that specifically say that you have the right to use the software but you just can't manipulate the source code to suit it to your own personal needs.
That's where free software comes in ...
Since I chose free software I chose freedom and now I can finally pump fresh air into my computer."