Modern medicine is taking greater and greater advantage of the diverse capabilities of ultrasound. High-intensity ultra-sound in particular is a promising technique for treating various medical conditions, but we still do not fully understand how the ultrasound induces physical changes in body tissue. Some ingenious experiments by Guy Delacrétaz and colleagues at Ecole Polytechnique Fédérale Lausanne in Switzerland, and Northwestern University in Illinois, US, have provided important new insights into the problem (Appl. Phys. Lett. 1997 70 3510). They have studied the effect of ultrasound in a tissue-like gel, and have found some surprising results.