。
Wood floats, but iron sinks.
Literal
Wood [topic-は] floats [but-が] iron [topic-は] sinks.
Minimal demonstration of contrastive は in a compound sentence — each clause has its own topic, joined by the contrastive conjunction が ('but'). Note the role of が here: it's not the particle but the connective 'but,' homophonous with the subject particle. Context and position disambiguate them.