I always loved science as a kid, and one of my favourite toys (aside from my Tasco telescope) was my microscope kit. The good thing about microscopes is that they can be used anytime, they're educational, and you can find endless things to study around the house and in the garden. Later on, its a good tool for studying practical biology.
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For children high powered microscopes are most popular as the kids can use them to see "invisible" things like blood cells, amoebas and bacteria. As well as all the tiny critters living in a drop of pond water. Do not buy a plastic "toy" microscope, as a poor quality microscope is totally useless, so you're just throwing your money away and will disppoint the child (same goes for telescopes with poor optics and cheap, flimsy mounts). Purchase a model with a built in light source. Models with mirrors are rarely sold today. There are no manufacturers of microscopes in the US - the best "economy" models are made in China. There are some very good products from China, but also some poor quality instruments, so be careful what you buy. A good microscope can be used all through to University and beyond (my parents are in their 70s and still use a simple microscope to check for parasites on the grass at their alpaca farm).