Triangle symbol on the bottom with a number inside.

THE NUMBER TELLS YOU THE CHEMICAL MAKE UP OF THE PLASTIC.....
1) Polyethylene terephalate (PET)
2) High density polyethylene (HDPE)
3) Unplasticised polyvinyl chloride (UPVC) or Plasticised polyvinyl chloride (PPVC)
4) Low density polyethylene LDPE
5) Polypropylene (PP)
6) Polystyrene (PS) or Expandable polystyrene (EPS)
7) Other, including nylon and acrylic
What you aren't told is that many of the plastics used are toxic and
the chemicals used to create a plastic can leach out of the plastic
and into the food / drink.