Another great question, jejones. I think the hardest things to understand are those which require us to step back from our everyday experience of the world and take a different perspective. Historically, people have a habit of seeing themselves and their perspective as different and special (for example, we used to think that the Earth was at the centre of the Universe) – real breakthroughs have only come when we’ve been able to imagine the world from a different perspective. Right now the biggest challenges (in my opinion) are understanding how time works (for example, why do we remember the past and not the future? how can seeing something happen change the future but not the past?) and how consciousness works (for example, do you experience the same thing as me when you look at something red, and what is this “experience” and where does it come from?).