The Science Show, ABC Radio National, Australia


Dunking’s dunking – right? Wrong! There is a science to dunking that explains why dunking a biscuit is quite different to dunking a donut.

Program Transcript

Robyn Williams: So Len, how do you dunk a donut?

Len Fisher: Very simple, if you think about what donuts are. Donuts are cooked in fat so donuts are waterproof: you could make an umbrella out of donuts if it wasn’t for the holes in the middle. Essentially waterproof, so if you dunk a donut – and Americans do it, the rest of the world doesn’t very much, but there’s a lot of Americans around, then it’s going to take a very long time for the liquid to get in. So how are you going to get the liquid in? It’s very easy, you bite a little hole in the bottom and then suddenly you’ve got the area exposed so the liquid can penetrate and away you go. We do very much the same sort of thing in reverse when we try to get oil out of porous rocks like sandstone and so on. We actually have to bash them up in order to get the oil out. But you can go a little further, you can start to get very scientific about donuts, and one of the many things that show up about science in the book is that, when you actually look at the science, quite often you’ll get a surprise. For example, with donuts you’ll find some donuts have great big holes – not the hole in the middle – when you bite into them you’ll see that they’re very porous like bread, which is why they hold together. But also you’ll find some that have got very big holes all through them and some that have got lots of smaller holes. And you think that the ones with big holes would hold a lot more coffee – they don’t. It’s the ones with the smaller holes but you have to do a little bit of science to find out and that little bit of science is actually very understandable, it’s not hard.

Robyn Williams: I suppose it’s better via the donut than via the rock that you’re trying to excavate and the oil well, which has got little to do with most kids. Do you find that is actually the case that if you go via the donut you get the science followed through, whereas you go through something else it doesn’t happen?

Len Fisher: Oh. yes indeed. I’ve had kids actually doing school projects and sending them to me for my comments just on this sort of basic thing, yeah. What I’ve done is just to go through the activities of an ordinary day and talk about how scientists might actually look at them. So of course you start with dunking, dunking a biscuit, dunking a donut: biscuits and donuts are different because biscuits actually dissolve, they’ve got sugar in them. We go through boiling an egg: you can work out from the book how to boil an emu egg.

Robyn Williams: What’s different about an emu egg, apart from size?

Len Fisher: It’s not apart from size, it’s is exactly size, but it’s the effect of the size on the boiling time that again gives you a scientific surprise. People for example in England, are often used to boiling small eggs. A nice soft boiled egg takes you about two and a half, two and three quarters minutes; hand them a big egg and they’ll to guess. But with the science you can actually see that when you go up 50% in the weight you have to more than double the boiling time to get the same consistency. Little tricks like that show up from the science because of the way that heat gets to the middle of the egg.

Robyn Williams: Do you know, I was once told way back that if I wanted to freeze my ice cubes more quickly I’d have to put hot water in the tray and this seemed to be completely ludicrous. Do you have a view on that?

Len Fisher: I’ve got a very strong view on that because I was never allowed to publish the work when I did it with CSIRO in the earlier days, round about 1970. It was called Mpemba’s ice cream wasn’t it, and it was the young African boy who had put his ice cream in the freezer and found that the hot mixture froze more quickly than the cold mixture. Now that just sounds crazy – in one sense, it is crazy. We’ve looked at it very carefully, there’s been a lot of papers published on this, but what we found was that because of the convection currents in the liquid that the onset of freezing actually occurs more rapidly in the hot than the cold, just because of the way the convection currents are set up. Once the whole thing is frozen it was rather different. But there’s always a little thing that you can find out, yeah.

Robyn Williams: That’s the first time in 30 years I’ve actually had a straight explanation of that, it’s quite extraordinary. I just thought it was voodoo, I just thought it was completely nonsense but then it turned up in the New Scientist with some sort of explanation like that. But when you take your book around and leaf through these examples, which sort really captures the imagination of a kid?

Len Fisher: It depends on the age of the kid, but you can actually choose the order of the chapters to suit yourself and the very last chapter, conventionally, is on the physics of sex and that’s the one that certainly grabs them the most. I’ve given many talks in high schools about that. I’ve told some of the stories from the book. I think one of my favourites was when I was giving a talk on the physics of sex and after it a young man about 14 came up and he wanted to ask a private question. So we stepped discreetly to one side from the audience, which I have to say consisted more of teachers than students. And his question was, if a girl’s a virgin does it go bang? So I gave him a very straight-faced lecture on the physics of drums.

Robyn Williams: Did it help?

Len Fisher: I very much doubt it but he certainly went away looking satisfied.

Robyn Williams: What about the other aspects of sex that you do include in the book?

Len Fisher: The sex one is a bit of a tricky one because one can’t look like a medical person, which I’m not, and one really can’t be offering advice. So what I’ve done there, and just for interest, is I’ve talked about sex from the point of view of the sperm, and the problems that a sperm has in swimming that incredible journey, which is something like 50 lengths for us of an Olympic swimming pool full of honey, and how it actually makes its way through. There’s a lot of little bits and pieces there, I’ve tried to lighten it up obviously rather than make it over serious.

Robyn Williams: You’re not worried in the slightest that you may be in fact just doing the sexy bits literally, and leaving out the hard stuff, so that instead of getting a challenge to the minds they’re getting, if you like, the easy way out?

Len Fisher: That’s a very fair question. No, I haven’t. At all stages I’ve made an effort to give the reader the real science because I think people do understand the real science. Yes, it takes a little bit of effort, yes, you have to work a little bit at working your way through, but heavens me, that’s the pleasure in learning and it doesn’t matter whether it’s science or anything else, it’s the pleasure in hobbies, for goodness sake. I mean, people who get interested in a hobby do get into the detail, do understand the detail and do get carried away by the detail just as I do in science. So I’ve tried to give people that and share what I do and what other scientists do.

Thanks are due to Robyn Williams and the ABC for permission to reproduce the full script of this interview.

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