Teacher Demonstration 14
Magic Hanky
Materials:
• Glass 1
• Water (add food colour if desired)
1
2
• Handkerchief (fine weave) 2
Use a rigid plastic cup if you are concerned about the safety of students repeating the demonstration with glass
Loose weave handkerchiefs will work but tend to let a few drops of water escape when turned
Instructions
1 Fill the glass with water so that the students do not suspect
you are tricking them with ‘trick water’
The glass does not need to be full to the brim
2 Cover the glass with the handkerchief
3 Push the centre of the handkerchief deep into the water so
that it becomes thoroughly wet
4 Stretch the top of the handkerchief taut across the rim by
pulling it down the outside of the glass – you should feel
some resistance and there should be no creases on the rim
Teacher Demo 14 – Magic Hanky | Page 1
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5 Turn the glass upside down and no water will leak (if one
or two drops escape, your handkerchief is probably too
loosely woven)
6 Remove the handkerchief to reveal nothing has changed –
pour the water into another glass or jug to prove you
haven’t
Teacher notes
This demonstration relies on the strong surface tension of water, the fine weave of the
handkerchief and atmospheric pressure.
A similar ‘trick’ can also be performed with a coaster (or piece of stiff cardboard) instead of a
handkerchief. Simply place the coaster on a cup of water, invert the cup with the coaster held
in place, and then remove the finger holding the coaster. The coaster will not fall and no
water will be spilt. For the water to leak out, the air bubble at the top (the inverted bottom) of
the cup would need to expand, which would create a vacuum (a region of lower than
atmospheric pressure). It may seem counterintuitive but it is the pressure of the atmosphere
pushing against the coaster, which prevents this vacuum from forming.
The ‘trick’ also works with a handkerchief because of the strong surface tension of water
which prevents it from passing through the tiny spaces between the threads of the
handkerchief. The same demonstration attempted with cheesecloth or other open-weave
cloths will probably result in at least a few leaked drops, if not many depending on the
looseness of the weave.
Teacher Demo 14 – Magic Hanky | Page 2
© 2005 | ABC Science Online
Nutrition and sugar in softdrinks
Students can be encouraged to make better choices by discussing the difference between
hunger and thirst. The following considerations may help your students make better choices in
the future:
• Our amazing bodies use various sensations to let us know all kinds of important things
such as when we are becoming too cold (which can lead to hypothermia and death), or too
hot or when we are injured, so that we avoid further injury
• When our body requires energy, we experience a hunger sensation (ie rumbling tummy)
• When our body requires water, we experience a thirst sensation (ie dry mouth)
• If you drink softdrink, fruit juice, milk or cordial when you feel thirsty, you are also giving
your body some energy which it wasn’t necessarily asking for (ie thirst is not the same as
hunger – you don’t eat because you’re thirsty and vice versa)
• If you are going to drink softdrink, make sure you don’t drink too much and remember
that a 250ml glass of softdrink contains about 7 teaspoons of sugar, a whole can contains
about 10, and a 600 ml bottle contains up to 18!
• Make sure you always do at least 30 minutes of physical activity every day, and more if
you can, and consider how much energy you’re putting into your body
• Take more notice of what you eat and how much energy your body is receiving through
food and drinks by reading nutrition labels and don’t forget that even healthy choices
such as fruit juice and milk contain energy!
• Consider the table below and calculate the fraction of the recommended daily energy
intake contained in one 600 ml bottle of softdrink (depending on age and gender, it
ranges from almost 10% to nearly 20%!)
Recommended Energy Intakes for Males and Females
Age
Kilojoules per day
Males
Females
6–7
7–8
8–9
10 – 11
11 – 12
12 – 13
13 – 14
14 – 15
15 – 16
16 – 17
18 – 30*
7900
8300
8700
9000
8100 – 9100
8300 – 9800
9200 – 10,300
9800 – 11,000
10,500 – 11,800
11,100 – 12,500
11,700 – 13,200
7100
7400
7700
7900
7300 - 8200
7700 – 8700
8100 – 9100
8400 – 9500
8600 – 9800
8300 – 9900
8800 – 10,000
Source: Recommended Dietary Intakes for Use in Australia – NH&MRC
* For Height 170cm / Weight 65.0kg
More information:
Australian National Health and Medical Research Foundation
Recommended Dietary Intakes for Use in Australia
http://www.nhmrc.gov.au/publications/diet/n6p4.htm#app2
Australian Bureau of Statistics
National Nutrition Survey: Nutrient Intakes and Physical Measurements Australia
http://www.abs.gov.au/Ausstats/[email protected]/0/95e87fe64b144fa3ca2568a9001393c0?OpenD
ocument
Safety notes
There are no specific safety notes for this demonstration, however you should always consider
the particular needs of your students and your classroom situation.
Teacher Demo 14 – Magic Hanky | Page 3
© 2005 | ABC Science Online