The influence of bubbles on sound has long attracted the attention of physicists. In his 1920 book1 Sir William Bragg described sound absorption caused by foam in a glass of beer tapped by a spoon. Frank S. Crawford described and analyzed the change in the pitch of sound in a similar experiment and named the phenomenon the “hot chocolate effect.”2 In this paper we describe a simple and robust experiment that allows an easy audio and visual demonstration of the same effect (unfortunately without the chocolate) and offers several possibilities for student investigations. In addition to the demonstration of the above effect, the experiments described below provide an excellent opportunity for students to devise and test explanations with simple equipment.

1.
W.
Bragg
,
The World of Sound
(
G. Bell and Sons Ltd.
,
1920
), p.
158
.
2.
F. S.
Crawford
, “
The hot chocolate effect
,”
Am. J. Phys.
50
,
398
404
(
May 1982
).
4.
Steps like this are very important in completing the learning cycle and are an integral part of ISLE, a philosophy of teaching and learning physics that mirrors scientific practice. See
E.
Etkina
and
A. Van
Heuvelen
, “
Investigative Science Learning Environment — A Science Process Approach to Learning Physics
,” in
Research-Based Reform of University Physics
, edited by
E. F.
Redish
and
P.
Cooney
(
AAPT
,
College Park, MD
,
2007
); online at per-central.org/per_reviews/media/volumel/ISLE-2007.pdf
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