The mercury I played with as a child was from batteries and thermometers............the real thing.........BTW, mercury is very heavy. I had a coffee can full of it in a place where I once worked. It was like being full of lead.........
Without dropping any more clues about my age, I will admit that I may have done the same. I do know that old thermometers and batteries used real mercury.
I'm not certain about battery applications, but thermometers are now using the gallium alloy:
"The take-home thermometer might be digital, chemical strips or a glass thermometer filled with a non-mercury liquid metal alloy.