Petsto grama is five hundred grams.
There is a plastic bottle lying on the ground. A young man from Hamburg picks it up because he wants to do something for the environment, is very proud of himself afterwards, notices it weighs almost nothing and shows it to his mum. Bottle and pride give you petsto, gram and mum give you grama.
The “pride” for sto is the same image as for one hundred – that is how the larger numbers are assembled from just a few blocks.