How do the BRITA water jugs know when the cartridges need changing when there is no connection between the lid and the filter?

[pattymc]

I might be very stupid but me and dh have been trying to work out how it works. If there are 4 people using the jug or 1 it still runs out at the same time so BIG waste of money I guess

they simply time 4 weeks or whatever it is so if there are fewer of you than 4 or if you've been on holiday then ignore it and carry on using it, that's what we do but then we are also quite cavalier with use by dates too!

I just ignore mine - there's only me and DC and it works it out by time (I think). I change it every 4-6 weeks