Just before a protest, your blood begins to pump through your body very strongly. As the time grows nearer, this increases. I have trained myself to remain calm, the same way people in the front line - policemen or firemen - can train their brains to slow down. But it takes a lot of concentration.
I came to Hong Kong to do a postgraduate degree, but I ended up staying and teaching English.
People expect teachers to be law-abiding. But Hong Kong is not democratic and that changes the game morally. If you get arrested for a completely peaceful demonstration, it makes the government look bad.
My first protest was in 2004. I ran on to a horse track dressed as a horse, with a T-shirt saying "Demand democracy now". It was the Hong Kong Cup, the biggest race of the year, and I delayed the start of the race by a few minutes.
Anyone can jump over a fence and run on to a piece of grass, but something about protesting makes it quite difficult psychologically: it felt like I was going into the boxing ring.
I put a lot of preparation into the exact details, tried to do it in a slow and controlled way. If you don't do enough preparation, things can get out of hand or turn violent.
I did another protest in 2005 on the anniversary of the Tiananmen Square massacre. I dressed in a Spiderman outfit and climbed on to a giant TV screen in central Hong Kong. I was given 21 days in prison.
I knew I was going to be arrested. But it is an unusual experience to be thrown in prison.
Hong Kong is different from mainland China: it has some degree of rule of law, some aspects of human rights. But having your freedom taken away is not an easy experience.
You have lots of time on your hands: one day feels like a week of normal life outside, so there is plenty of time to think. There were a few people I could chat to, but most were hardened organised criminals. It was a pretty unpleasant experience. But you get through it.
Obviously, I weighed up whether or not to do it again. I studied the law, to get a sense of what my sentence would be. I knew it would be a matter of months, but I decided I could handle it.
The last protest I did was a pretty big one. I hung banners on the main bridge in Hong Kong - the bridge to the airport - during the Olympics. I had to prepare for a few months - get the equipment together, visit the location and think about the kind of impact it would have. They had to close the bridge.
I was given a six-month sentence, but it was cut by a third for good behaviour. I lost 30kg in those four months.
Most schools in Hong Kong are very conservative and I have gone through quite a lot of teaching jobs since 2004. At the moment, fortunately, my headteacher is with me politically. But most are afraid of getting in trouble for employing someone with a conviction.
I was asked to leave one school because the head was worried what more conservative parents would say. Parents worry that if teachers break the law, their children will, too. It's pathetic, but some people are like that.
The first thing I say to students is: "You don't have to agree with everything I say." I strongly urge them to have some debate with me. I don't care about their politics - I just want them to think for themselves and have their own opinions.
Every teacher has a constant struggle: whether to conform with the establishment or whether to be a bit more creative and take risks. You don't necessarily have to be arrested, but I would encourage teachers to take more risks.