20.Section 11 makes it an offence for a person (A) aged 18 or over intentionally to engage in sexual activity (as defined in section 78), in order to gain sexual gratification, when a child aged under 16 is present or in a place from which A can be observed. Where the child is aged 13 or over but under 16, the prosecution must prove that A did not reasonably believe that he was 16 or over. The offence is committed if A knows or believes that the child is aware that he is engaging in the activity or intends that the child should be aware of this. This offence would cover, for example, A masturbating himself in front of a child, or A masturbating himself in the presence of the child to whom he is describing what he is doing, perhaps because the child is covering his face. It would also cover the situation where A performs a sexual act in a place where he knows that he can be seen by a child, for example via a webcam.