Watched the Horizon programme "What on Earth is Wrong with Gravity?" last night. There was an interesting bit in it about GPS clocks and how gravity effects them. I must confess, particle physics is not my forte (to put it mildly!) and I got a bit lost with Einstein, time dilation and curved space time. My science teacher husband was engrossed, but I had a bit more background to assimilate before I really 'got' the issue with time and gravity.
Found this blog earlier which had a fascinating debate about GPS clock rates. I sort of kept up with the flow initially but lost it once red shifts came into play. I'm sure it's quite straight-forward really; it's just something I've never really thought about.