If a picture is worth 1,000 words, than an animated gif must be worth at least 5,000!
Google Slides supports animated gifs, and it’s an easy way to liven up a presentation. Sure, you could insert a Youtube video, but sometimes that is just plain overkill. Unfortunately, you have no control over the gifs, and they will endlessly loop.
To find animated gifs to use, try the Search Tools options in Google Image Search.
By selected animated, the search results will be animated gifs that you can use in the classroom. There are other search tools, such as Giphy, that also let you search for animated gifs, but the problem there is a higher risk of stumbling across an inappropriate picture. A lot of school districts have Google Save Search turned on to alleviate this issue. (Nothing is absolute, teacher guidance is always necessary.)