Just a regular 1 liter plastic bottle (≈ 33.81 oz). I noticed that when you gently tap on the side of the bottle, it gives off a pleasant low-pitched tone, almost like some kind of a bass drum, which is why I thought it could be used as a low-key percussive element in your Caustic creations. Alternatively, you could mangle it beyond recognition by time-stretching, bitcrushing and/or saturating it and filtering it through a multitude of other effects.