A plankton sized piece of information . . . but in our house the kettle is of vast importance. This particular junk shop item has been with us for about ten years and has had many other 'lid knobs' in its time.
I think the original one was probably bakelite ( polyoxybenzylmethylenglycolanhydride) apparently, then followed a series of corks that disintegrated, then a rather beautiful brass curtain rail finial which an be seen - if you really have nothing else to do - on my post-materialism site; then . . . er, ah yes, a lead penguin, then this present lead sheep although without its modernistic wire construction - this added when the wire wrapped around belly design finally failed.
Unless the vessel itself melts through being boiled dry - something my mother excelled in, I think it will be with us for ever, the knob being changed every six months or so when the last one fails, or something else presents itself as being a super alternative.