I'm not high, and I can't see anything but a lake.

Edit: wait, is it a wall?

Spoiler: It's a fence in need of a power wash. The giveaway is the fact that the "shadows" on the "lake" don't match the source. Hold the photo upsidedown to get your brain to snap out of it. Afterwards you can't unsee it.

Yeah me too but what about the monkey?

Even knowing it's a wall my brain keeps wanting to see a lake

That's an odd looking zoo... What's to stop the monkey from just climbing up the tree and over the wall?

I wasn’t even high and I thought it was a wall

Optical illusion is when someone steals my glasses.