Forrest didn't have the SLIGHTEST idea about a potion ingredient, nor did he have much time to find out what was used in potions. Being the muggleborn he was, he knew muggle medicine was made of herbs, so he'd gone directly to the lake. He was panting from all the running, and there he plopped down somewhere and opened the book to maybe find something that he recognised.

Nothing... all the names were foreign to him--or so they seemed in his panic. He turned the pages quickly, eyes always on the ingredients part, but Chinese would have made more sense than those wizarding names. He closed the book in panic and looked around as if something would shine like in those video games, when you had to collect an item. He kept looking around, with a desperate expression, but nothing came alive...

The grass... the lake water... a wild daisy and--and--he found a dead stick that must have fell from a tree nearby. Those would be his ingredients. It was better than returning empty handed, yes?

Yes.

Pulling a handful of grass, he shoved it into his hands, as well as the dead stick and that daisy. He didn't have a container for the lake water, so he quickly ran to the castle and down the potions classroom to get his water bottle.

...

After getting the bottle from the classroom, he returned to the lake. He drank half of the water inside and emptied the part he couldn't drink into the lake. Then, walking a few metres forward so that the water he'd emptied there wouldn't get inside the bottle. Filling it up with the lake water, he closed the lid carefully and ran back to the castle.