Hi Jacqueline.
I have found this as well, i have just got my 2 year old on a beaker at the same time as my 11 month old and i have gone through 4 bottle style and now have 5 of another kind with a beaker style teat which have lasted longer. I did notice my youngest sticking his finger in the top. So i have bought at least 9 beakers/bottles over the past 3 weeks at £3.50 a go, as you cant just buy the teats. I think they are just a rip off and mr tommee tippee has a letter arriving shortly.