Tommee Tippee identified this from boiling water being poured directly into a bottle, where the lid and teat have been placed straight back on, as a vacuum can build inside the bottle. This vacuum can potentially cause the bottle to suck inwards resulting in the bottle becoming misshapen. In addition to this, be cautious not to use a cold bottle from the fridge, pouring boiling water straight into it. WHO (World Health Organization) guidelines advise to allow freshly boiled water to cool for around 30-minutes before making up a feed. This method would prevent a vacuum being caused when the lid and teat is placed back.