On 26th March 2019, an open STEM lesson called ‘Blood Spatter – Be a Crime Scene Investigator’ was conducted for F.2A. It was one of the STEM activities suggested under the Professional Development Schools (PDS) Scheme, led by Maryknoll Fathers' School. This particular open lesson covered topics in Mathematics, Integrated Science and ICT. More than 10 teachers from different schools were involved and joined the lesson as observers. The students were put into groups, did measurement on bloodstains, obtained angles of projection using knowledge of trigonometry and set up projection lines for locating the position of the point of impact. Finally, by means of mathematics software, they could modify the accuracy of the best position of the point of impact. The lesson was conducted successfully and the students really enjoyed it.