The theme is echoed by the head-to-head nature of the gameplay (land and sea in opposition), the semi zero-sum nature of the score (the fact that when one person lets plastic past their line the other person benefits due to the obstacle no longer being in place, as well as the point difference) and the projectiles themselves, which compromise a variety of fairly opposing themes, e.g. enlarge and shrink, shield and explosion, gold and poo.  Also, some are good for you, bad for your opponent and the balance and behaviour of the projectiles is designed so that sometimes it can be in your best interest to bounce back both a good and a bad object - especially since some projectiles increase their velocity between bounces more so than others.