Any soft polymer will slowly deform under constant pressure over time ... so will glass. Thats why they're called 'plastics', they 'flow' and dont 'rebound' under the right conditions - the term plastic deformation (reduced 'rebound') as opposed to elastic deformation (complete 'rebound'). In actuality plastics are a semi-solid and can be considered as a 'specialized liquid' state rather than a 'true solid'.