Ahoy hoy Hugo et al.,
Modification: a house Open source is like buying a house and being free to decorate it however you want, to build extensions or demolish walls. Closed-source software strictly limits what you can do with it.
One of the best metaphors I've heard so far regarding modification (i.e. derivative work) comes from my dear mother, who is quite into gardening and roses. So.
Modification: Flowers for your garden
Free Software is like buying flowers for your garden. If the seed of those flowers spreads and you get more flowers, you can give those seeds or flowers away if you want to. Proprietary software is like signing an agreement that you will never ever use or give away those seeds or flowers.
And accordings to my mother, in the gardening community, such an idea would be considered madness and an insult to the craft. ;)
Kind regards, Simon