Properties that plastics and glass always share

1. Both of them can melt and form into various shapes
2. They both are recyclable and cheap (somewhat cheap, depending on the 'trend')
3. The majority of them (if not all) are transparent.

These are some similarities amongst the properties of plastic and glass. Hope I helped. 

Both can melt into different things. Also, Both CAN be clear.

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water