Explain that this is usually OK, but sometimes you might need to check that it's not overheating. List ways to keep the computer cool. Mention how to check the CPU/system temperature (probably have to use the terminal).

Some people may be worried that there is a health risk in having a hot computer on their lap. I think sperm start dying-off above 14 degrees C, so it seems plausible that there could be at least a short-term impact on male fertility. If you're feeling up to it, point to (and briefly summarise) some decent web resource on the issue.