Once again, borrowing from the Play at Home Mom site, I pulled out the baking soda and vinegar. And to make it even more inquiry based, I colored the vinegar with food dyes so they had some primary colors to work with. Now that we have been doing this sort of open ended play invitation for a little over a year, they boys jumped right in and started as soon as everything was out.
They got to use the medicine syringes to pull up the vinegar and then squirt it out onto dishes of baking soda. This was a great activity that held their attention for 40-50 minutes (until we ran out of baking soda and vinegar!) They were really excited to see the foaming and hear the noises it made. Then they started mixing colors and the play took on a whole new direction. They were eagerly seeing what colors they could make.
When all the vinegar was used up they started stirring with spoons and were surprised to find the baking soda was gone! When we decided it was time to clean up and start making dinner they both happily helped pick up the dishes and bring them to the counter.