Most of the teachers I have met in my roles as a science educator, have expressed that the most difficult part of a science lesson is having everything they need, when they need it. For an Elementary Science Lesson, the materials you need are not all that difficult to come by, most can be found at the grocery store. If you are prepared and organised, an impromptu science lesson should be no problem. I found that the following article has some great advice on how to do this and I also have a few suggestions of my own.

1) Once you have found a lesson that works well, buy all the supplies you need and prepare a lesson box. Be sure you try all activities yourself first so you know how to troubleshoot in class if necessary.

2) Make sure you clearly label the box and include inside a copy of your lesson plan, any student worksheets or instructions, and a list of materials/amounts so you can restock when needed. The linked article suggests a shoebox for each student, but I would have a large box for each science lesson/topic and have small shoeboxes or baskets stacked away, so that you can fill and hand out to each table with enough materials for students to work in groups.

3) Keep the lesson box in a communal area so that all teachers have access to the materials. The advantage of having a large lesson box is that you can have a few different variations of the lesson plan for different year levels and the box can be shared throughout the school saving on costs for supplies.

If all of this still seems too much for you, there are different organisations that will do the hard work for you. Many universities or large science companies with an education division offer science lessons or teacher resources, so have a quick search for organisations near you!