10. Why Does Batman Have A T-Rex And Coin In The Batcave?
There are actually two kinds of questions here; the first has an obvious answer in that there's a canonical explanation for why Batman keeps trinkets of his various encounters - most hailing from Dick Sprang's classic Batman stories - but the second, i.e. the 'how' the Dark Knight managed to get these souvenirs into the Batcave, is a tad more confusing.
For almost as long as there has been a Batcave Batman has kept souvenirs of his exploits in it. The T-Rex and the Penny in particular are the standout attractions, and they both - unsurprisingly - originate from some of Bruce Wayne's earliest adventures as the Dark Knight.
Of course, with this being DC, there have been multiple interpretations of the same origin, which we're thankfully able to split up into pre and post-Crisis continuity. The New 52 and DC Rebirth muddy the waters further, but we'll just go with the most recent versions as the canonical explanations to why - and how - Bruce came into possession of his iconic souvenirs.
First the T-Rex. The origin for how Batman acquired the robotic dino came in Batman Chronicles #8, where it's revealed that Bruce took it from a theme park ran by an old friend of the Waynes. A business partner uses it for evil, and once a slightly melancholic Batman rains on his parade, dejected at the unending nature of his war against crime, Alfred suggests that he keeps trophies in the cave to remind him of his successes. So he does.
Now for the penny. Given there's only one Batman villain who incorporates a coin into his gimmick, it's obvious that Two-Face has something to do with it, right?
Only he didn't... because it's actually the Penny Plunderer responsible for this contraption. The New 52 is the most recent story to provide an explanation for the souvenir, so it's the one we'll go with.
Back in Zero Year, a young Batman goes up against the Riddler. The penny is an outdoor piece of art that Lucius Fox uses to bounce signals past Riddler's jammers, and then both presumably decided to move the contraption to the Batcave.